rework help messages for fee-related options

- mention the units and show the default for -paytxfee
Add DNS seed of open-nodes.org
[Qt] New status bar Unit Display Control and related changes.

- New status bar control shows the current Unit of Display.
  When clicked (left,or right button) it shows a context menu
  that allows the user to switch the current Unit of Display (BTC, mBTC, uBTC)
- Recent Requests and Transaction Table headers are now updated when
  unit of display is changed, because their "Amount" column now displays the
  current unit of display.
- Takes care of issue #3970 Units in transaction export csv file.
- Small refactors for reusability.
- Demo Video https://www.youtube.com/watch?v=wwcr0Yh68go&list=UUG3jF2hgofmLWP0tRPisQAQ
- changes after Diapolo's feedback. Have not been able to build after last pool, issues with boost on MacOSX, will test on Ubuntu these changes.
- removed return statement on switch
- renamed onDisplayUnitsChanged(int) to updateDisplayUnit(int)
- now getAmountColumnTitle(int unit) takes a simple unit parameter. moved to BitcoinUnits.
small cleanup of #ifdefs in BindListenPort()

- SO_NOSIGPIPE isn't available on WIN32 so merge the 2 non-WIN32 blocks
- use predefined names from header for IPV6_PROTECTION_LEVEL and
  PROTECTION_LEVEL_UNRESTRICTED
add missing BOOST_FOREACH indentation in ThreadSocketHandler()
add OpenSSL RAND_cleanup() on OpenSSL shutdown

- to securely erase the memory used by the PNRG
- also rework 2 comments in OpenSSL init
Use async name resolving to improve net thread responsiveness

In the LookupIntern(), things changed are:

  1. Call getaddrinfo_a() instead of getaddrinfo() if available, the former is a sync version of the latter;
  2. Try using inet_pton()/inet_addr() to convert the input text to a network addr structure at first, if success the extra name resolving thread inside getaddrinfo_a() could be avoided;
  3.  An interruption point added in the waiting loop for return from getaddrinfo_a(), which completes the improve for thread responsiveness.

A easy way to see the effect is to kick off a 'bitcoind stop' immediately after 'bitcoind -daemon', before the change it would take several, or even tens of, minutes on a bad network situation to wait for the running bitcoind to exit, now it costs only seconds.

Signed-off-by: Huang Le <4tarhl@gmail.com>
RPC client: Simplify command line string-to-JSON-value conversion code

By default, all command line parameters are converted into JSON string
values.  There is no need to manually specify the incoming type.

A binary decision "parse as string or JSON?" is all that's necessary.

Convert to a simple class, initialized at runtime startup, which offers
a quick lookup to answer "parse as JSON?" conversion question.

Future parameter conversions need only to indicate the method name
and zero-based index of the parameter needing JSON parsing.
CBloomFilter::clear() method
Relay double-spends, subject to anti-DOS

Allows network wallets and other clients to see transactions that respend
a prevout already spent in an unconfirmed transaction in this node's mempool.

Knowledge of an attempted double-spend is of interest to recipients of the
first spend.  In some cases, it will allow these recipients to withhold
goods or services upon being alerted of a double-spend that deprives them
of payment.

As before, respends are not added to the mempool.

Anti-Denial-of-Service-Attack provisions:
 - Use a bloom filter to relay only one respend per mempool prevout
 - Rate-limit respend relays to a default of 100 thousand bytes/minute
 - Define tx2.IsEquivalentTo(tx1): equality when scriptSigs are not considered
 - Do not relay these equivalent transactions

Remove an unused variable declaration in txmempool.cpp.
UI to alert of respend attempt affecting wallet.

Respend transactions that conflict with transactions already in the
wallet are added to it.  They are not displayed unless they also involve
the wallet, or get into a block.  If they do not involve the wallet,
they continue not to affect balance.

Transactions that involve the wallet, and have conflicting non-equivalent
transactions, are highlighted in red.  When the conflict first occurs, a
modal dialog is thrown.

CWallet::SyncMetaData is changed to sync only to equivalent transactions.

When a conflict is added to the wallet, counter nConflictsReceived is
incremented.  This acts like a change in active block height for the
purpose of triggering UI updates.
Add -respendnotify option and new RPC data

-respendnotify=<cmd> Execute command when a network tx respends wallet
tx input (%s=respend TxID, %t=wallet TxID)

Add respendsobserved array to gettransaction, listtransactions, and
listsinceblock RPCs.  This omits the malleated clones that are included
in the walletconflicts array.

Add RPC help for respendsobserved and walletconflicts (help was missing
for the latter).
Add release notes entry
Formatting, spelling, comment fixes.
Move non-trivial uint256.h methods to uint256.cpp
fix RPC error replies

After pull #4288, RPC messages indicating errors have a Content-Length unrelated
to their actual contents, rendering bitcoin-cli and curl unable to decode the
reply.

This patch sets the Content-Length field based on the actual content returned.

Additionally, pull #4288 clobbered the error descriptions provided in
ErrorReply, which bitcoin-cli relies upon; this patch moves #4288 http-error
descriptions to an HTTPError method, allowing HTTPReply to pass content on
unchanged.
Track peers' available blocks
Add a skiplist to the CBlockIndex structure.

This allows fast (O(log n)) access to far predecessor blocks.
Use it to speed up CChain::FindFork and CChain::GetLocator.
Add skiplist unit tests
Merge pull request #4440

16f33f1 fix RPC error replies (kazcw)