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.
Reduce RippleD dependencies on Beast:
* Use static_assert where appropriate
* Use std::min and std::max where appropriate
* Simplify RippleD error reporting
* Remove use of beast::RandomAccessFile
Reduce Beast public interface by using std:: alternatives:
[FOLD] Remove Range.h
[FOLD] Remove LinkedListPointer
[FOLD] Remove ScopedValueSetter
[FOLD] Remove Win32 Registry Stuff
[FOLD] Remove FileSearchPath
[FOLD] Trim SystemStats down
[FOLD] Remove beast::static_bassert
[FOLD] Remove BinaryEncoding.h
[FOLD] Remove BinaryEncoding
[FOLD] SystemStats
[FOLD] Remove UnsignedInteger/UnsignedIntegerCalc
[FOLD] Use std::string instead of beast::String
[FOLD] Remove gratuitious use of beast::String