Issue #65: Give the correct error when wallet is locked

When sending coins to an address with a wallet that's both
locked and has insufficient funds, the correct ErrWalletLocked
error will be returned.
Implement a batching rescan manager.

Recent btcd versions only allow one rescan to run at any given time
per websocket client.  To better handle this, a new set of goroutines
are started by the account manager which batch and serialize rescan
jobs.

If no rescans are currently running, a new rescan starts.  If a rescan
is already being processed, the request is queued and runs after the
current rescan finishes.  For any additional incoming requests before
the current rescan finishes, the requests are merged with the
currently-waiting request so both can be handled with a single rescan.

This change also prepares for rescan progress notifications from btcd,
but are still unhandled until the necessary details for
partially-synced addresses are added to the wallet file format.
Avoid unnecessarily calculating a rescan job twice.
Make a comment understandable.
Support partial syncing of addresses in wallet format.

This change reappropriates the unused `last block` field from Armory's
wallet format to hold the block chain height for a partially synced
address, that is, an address that has been partially synced to
somewhere between its first seen block and the most recently seen
block.  The wallet's SyncHeight method has been updated to return
partial heights as well.

The actual marking of partially unsynced address from a rescan
progress update is not implemented yet.
Simplify logic in WalletRequestProcessor.
Mark partially synced addresses.

This change adds a notification handler for the new rescanprogress
notification and takes advantage of the recent rescan manager and
partial syncing support to mark addresses as partially synced.  If the
network connection to btcd is lost or wallet is restarted during a
rescan, a new rescan will start at the earliest block height for any
wallet address, taking partial syncs into consideration.
Return correct JSON object for listunspent.

This change fixes the reply for listunspent to return a JSON object in
the same format as done by the reference implementation.  Previously,
listunspent would return an array of the same objects as returned for
listtransactions.
Implement addmultisigaddress and createmultisig.
rework the account manager somewhat.

- Move the MarkAddresForAccount and LookupAccountByAddress functionality
into account maanger.

- Move the wallet opeing logic into account manager (the only place that calls
it) and unexport.

- Move accountHandler to using a single channel for commands. Many of
the commands have ordering restraints (add account, list all accounts,
remove account, access account, mark account for address) which are very
much undefined with the multi-channel model.

- Rework all callers of LookupAccountByAddress to get the account structure
directly.
fix up validateaddress to use a btcjson.ValidateAddressResult.
Also fix setting the account and hex fields.

ok @jrick
Remove some unused vars.
Report correct change address after composing txs.
Kill last MarkAddressForAccount call and func.
Remove old, commented-out code.

Found by @davecgh.
Use btcjson result type for listunspent.
Delay JSON unmarshaling until needed.

This change takes advantage of the RawMessage type in the
encoding/json package to defer unmarshaling of all JSON-RPC values
until absolutely necessary.

This is particularly important for request passthrough when btcwallet
must ask btcd to handle a chain request for a wallet client.  In the
previous code, during the marshal and unmarshal dance to set the
original client's request id in the btcd response, large JSON numbers
were being mangled to use (scientific) E notation even when they could
be represented as a integer without any loss of precision.
Return a btcjson result type for createmultisig.

ok @jrick
Add a comment for FinishUnmarshal.
Rework wallet apis somewhat.

- Instead of returning a special constructed type whenever queries for an
address.  Return the internal object with an immutable external
interface.

- Make the private key gettable from PubKeyAddress to prevent having to look up
multiple times to get information from the same structure

- Enforce addresses always have public keys.