Signature canonicalization and transaction mutation fixes
Permit memos up to 1Kb.
* Overlong memos are basically treated like invalid signatures.
* Ability to clear message key.
Consolidate C++14 compatibility into new headers
Remove unused TextDiff
PeerFinder fixes:
* Fix local advertisement (was missing)
* Fix Livecache histogram display
* If no [ips] are specified, use r.ripple.com
* Use different backing stores for PeerFinder and Validator databases
Fix the ledger range we advertise to peers
Fix beast::asio failures on OS X.
There are 38 unittest failures on OS X.  These changes address all of
them by adjusting which side of the socket (send or receive) gets
shut down.  In each case, the failure was 'Socket is not connected'.
I've interpreted that to mean that the other thread had already shut
down its side of the connection.
Set version to 0.22.0