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>
New classes and types for handling asset amounts:
  * Quality opaque type for order book quality
  * Amount replacement for STAmount
  * Amounts, in/out amount pair for offers
  * 'core' namespace with type aliases for Ripple primitives.
New classes for processing offers in order books:
* BookTip provides consume-and-step offer traversal
* OfferStream applies offer business rules and presents offers to callers
* Taker class manages state for the active party during order processing
* Offer class wraps book offers for presentation
Add experimental offer cross templates:
  * Also includes experimental SConstruct changes
[WIP] Autobridge test implementation
Use new OrderBook code
[FOLD] Fix Quality unit tests