Sub in std::array for boost::array and beast::FixedArray
Force BOOST_ASIO_HAS_STD_ARRAY to 1
for the clang / gcc libstdc++ 4.8 combination
Remove unused legacy code
Add bg_threads and high_threads RocksDB tunables
Configuration file improvements:
* Set default node_db to RocksDB from HyperlevelDB, to match gateway
  appliances and Ripple Labs server configurations.
* Set some rpc_startup options to reduce logging and save disk space.
* More sensible documentation regarding WebSocket SSL directives

Signed-off-by: The Doctor <drwho@virtadpt.net>