Improve transaction security
* Check signatures of every transaction on every validator
* Remove obsolete code
* Check transaction status in submit/sign RPC handler
Set version to 0.26.3-sp2
Use trusted validators median fee
Set version to 0.26.3-sp4
New class RPC::Status enforces JSON-RPC 2.0 error format.

* Relevant issues:
  * RIPD-92
  * RIPD-97
  * RIPD-98
  * RIPD-439
Fix outgoing bytes calculation in HTTP server.
Call Stoppable::stopped in PeerFinder onStop.
Disable transaction submission tests under Travis.
Refactor SerializedTransaction:
* Use boost:tribool instead of two intertwined bool variables
* Trim down public interface, reduce member variables
Disable SSLv3
Remove boost_thread dependency (RIPD-216).

Fixes RIPD-216
Fix tabs and trailing whitespace.
Detab beast
Fix URL compositing in Beast (RIPD-636).
RippleSSLContext.cpp cleanup.

* These cleanups precede work on RIPD-108.
Return descriptive error message from memo validation (RIPD-591).
Fix dependency on boost::thread on OS/X.
Workaround for MSVC move special members.
Refactor STParsedJSON to parse an object or array [RIPD-480]

The implementation of multi-sign has a SigningAccounts array as a
member of the outermost object.  This array could not be parsed
by the previous implementation of STParsedJSON, which only knew
how to parse objects.  This refactor supports the required parsing.

The refactor divides the parsing into three separate functions:

 o parseNoRecurse() which parses most rippled data types.
 o parseObject() which parses object types that may contain
   arbitrary other types.
 o parseArray() which parses object types that may contain
   arbitrary other types.

The change is required by the multi-sign implementation, but is
independent.  So the parsing change is going in as a separate
commit.

The parsing is still far from perfect.  But this was as much as
needs doing to accomplish the ends and mitigate risk of breaking
the parser.
Add "deferred" flag to transaction relay message

If we receive a deferred transaction from a server in our
cluster, treat it as if it wasn't received from a server
in our cluster.

This currently has no effect but is needed for server to
interoperate with future code that will relay deferred
transactions.