Update for recent btcchain ProcessBlock API change.

Now that the ProcessBlock function returns whether or not the block was an
orphan, the code which requests the parent blocks from the peer that sent
them has been moved to directly after the call to it.

This makes the handling more obvious and has allowed removal of the
blockPeer map which was kept so the notification handler could identify
which peer to request the blocks from.

ok @jrick
Update for btcchain ProcessBlock behavior flags.

ok @jrick
Update addblock for ProcessBlock behavior flags.
Update for SubmitBlockOptions field name change.
Expose a close ntfn channel to all RPC handlers.

This commit modifies the RPC server such that all handlers now receive a
channel which will be notified when a client disconnects.  This
notification can then be used to stop long-running operations early when a
client disconnects.

This capability was already present for websocket clients, but this commit
exposes it to standard HTTP clients as well.
Modify NewBlockTemplate to accept nil pay addr.

There are certain cases such as getblocktemplate which allow external
callers to be repsonsible for creating their own coinbase to replace the
generated one.  By allowing the pay address to be nil in such cases, the
need to specify mining addresses via --miningaddr can be avoided thereby
leaving the payment address management up to the caller.
Update for recent btcjson changes.

This commit updates the types to match the recent changes to the btcjson
result types.
Make hex decoding consistent in RPC handlers.

The hex package requires an even number of characters in hex encoded
strings.  Some of the handlers already prepended a zero if necessary to
make this condition true, however other did not.  This commit extends this
functionality to all handlers and also makes the error return consistent.

ok @jrick
Split imports into logical groups
golint -min_confidence=0.3 .

This commits removes a number of golint warnings. There is a class of
warnings which I can't fix due to unsufficient knowledge of the domain
at this point. These are listed here:

addrmanager.go:907:1: comment on exported method AddrManager.Attempt
should be of the form "Attempt ..."
addrmanager.go:1048:1: exported function RFC1918 should have comment or
be unexported
addrmanager.go:1058:1: exported function RFC3849 should have comment or
be unexported
addrmanager.go:1065:1: exported function RFC3927 should have comment or
be unexported
addrmanager.go:1073:1: exported function RFC3964 should have comment or
be unexported
addrmanager.go:1081:1: exported function RFC4193 should have comment or
be unexported
addrmanager.go:1089:1: exported function RFC4380 should have comment or
be unexported
addrmanager.go:1097:1: exported function RFC4843 should have comment or
be unexported
addrmanager.go:1105:1: exported function RFC4862 should have comment or
be unexported
addrmanager.go:1113:1: exported function RFC6052 should have comment or
be unexported
addrmanager.go:1121:1: exported function RFC6145 should have comment or
be unexported
addrmanager.go:1128:1: exported function Tor should have comment or be
unexported
addrmanager.go:1143:1: exported function Local should have comment or be
unexported
addrmanager.go:1228:2: exported const InterfacePrio should have comment
(or a comment on this block) or be unexported
discovery.go:26:2: exported var ErrTorInvalidAddressResponse should have
comment or be unexported
limits/limits_unix.go:19:1: exported function SetLimits should have
comment or be unexported
limits/limits_windows.go:7:1: exported function SetLimits should have
comment or be unexported
util/dropafter/dropafter.go:22:6: exported type ShaHash should have
comment or be unexported
util/dropafter/dropafter.go:38:2: exported const ArgSha should have
comment (or a comment on this block) or be unexported
util/dropafter/dropafter.go:128:5: exported var ErrBadShaPrefix should
have comment or be unexported
util/dropafter/dropafter.go:129:5: exported var ErrBadShaLen should have
comment or be unexported
util/dropafter/dropafter.go:130:5: exported var ErrBadShaChar should
have comment or be unexported
util/showblock/showblock.go:24:6: exported type ShaHash should have
comment or be unexported
util/showblock/showblock.go:46:2: exported const ArgSha should have
comment (or a comment on this block) or be unexported
util/showblock/showblock.go:163:1: exported function DumpBlock should
have comment or be unexported
util/showblock/showblock.go:211:5: exported var ErrBadShaPrefix should
have comment or be unexported
util/showblock/showblock.go:212:5: exported var ErrBadShaLen should have
comment or be unexported
util/showblock/showblock.go:213:5: exported var ErrBadShaChar should
have comment or be unexported
Avoid reallocs in removeDuplicateAddresses
Replace map[a]bool with map[a]struct{}

The later uses no memory storage for values and provides the same
functionality.
Use chan struct{} for semaphores

With semaphores we don't actually care about the value passed in. It
makes sense to use a 0 bytes type in these cases.
There is also the added benefit of compiler optimisations for this
specific use case as described here:
https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub
Implement validateaddress.

validateaddress will simply return whether the given address is
valid or not.  For the full implementation, one should query
btcwallet instead.
Don't log error if getrawtransaction finds no tx.

ok @davecgh
Log an error when json encoding of peers.json fails.
Correct getdata throttling.

This commit corrects an issue where the data requested by getdata was not
being properly throttled which could lead to higher than desired memory
usage on large requests.
Remove disconnect on unrequested transaction.

BitcoinJ, and possibly other wallets, don't follow the spec of sending an
inventory message and allowing the remote peer to decide whether or not
they want to request the transaction via a getdata message.  Unfortuantely
the reference implementation permits unrequested data, so it has allowed
wallets that don't follow the spec to proliferate.

While this is not ideal, this commit removes the functionality which
disconnects peers for sending unsolicited transactions to provide
interoperability.
Add gettxout to btcctl.
Lookup each DNS seed in its own go routine.

By putting each DNS seed in its own go routine, btcd can start connecting
to nodes as they are found instead of waiting for all seeds to respond.  This
significantly speeds up startup time.

Additionally, logging was added to show how many addresses were fetched from
each seed.