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
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.
Fix OS X version parsing/error related to OS X 10.10 update.
Change NodeStore::Backend tests to reflect observed patterns:
Empirical evidence shows a database access pattern with few hits
and many misses (objects that don't exist). This changes the timing
tests so they more accurately reflect rippled's actual usage:
* Add read missing keys test
* Increase numObjectsToTest to 1,000,000
* Alter PredictableObjectFactory to seed RNG once only
* Make NodeStoreTiming a manual test
Use autotls instead of multitls in websocket:
The MultiSocket class implements a socket that handshakes in multiple
protocols including SSL and PROXY. Unfortunately the way it type-erases the
handlers and buffers is incompatible with boost::asio coroutines. To pave the
way for coroutines this is part of a larger set of changes that roll back the
usage of MultiSocket to older code, and some custom implementations that use
templates. The custom implementations are more simple since they use
coroutines. Removing MultiSocket will make many other classes and source files
unused, a big win for trimming down the codebase size.
Workaround for MSVC stdlib and coroutine interaction:
If beast::Time::currentTimeMillis is first called from a coroutine launched
using boost::asio::spawn, Win32 throws an exception. This workaround calls
getCurrentTime once in main to prevent the exception.
Reference:
    https://svn.boost.org/trac/boost/ticket/10657
Add beast::asio::ssl_bundle workaround:
This works around the limitation that 1.56 boost::asio::ssl::stream objects
do not support r-value move or construction. It is required when the stream
does not own the socket.
Return error_code from beast::http::basic_parser:
This changes the HTTP parser interface to return an error_code instead
of a bool. This eliminates the need for the error() member function and
simplifies calling code.
Keep a list of section config values that are not key/value pairs:
This change to BasicConfig stores all appended lines which are not key/value
pairs in a separate values vector which can be retrieved later. This is to
support sections containing both key/value pairs and a list of values.
Remove HTTP::ScopedStream:
This class was used to allow stream style operator<< to write to the
HTTP::Session. This is being superceded by a more robust object-based model
that supports coroutines.
New JsonWriter for improved client performance (RIPD-439):
When JSON-RPC and Websocket responses are calculated, the result is stored
in intermediate Json::Value objects and later composed in a single linear
memory buffer before being sent to the socket.  These classes support a
new model for building responses that supports incremental construction
of JSON replies in constant time and removes the requirement that all
data returned be located in continuguous memory.
* New JsonWriter incrementally writes JSON with O(1) granularity and memory.
* Array, Object are RAII wrappers for the O(1) JsonWriter.