Tidy up some STAmount and base_uint operations:
* Constrain use of arithmetic operators in STAmount
* Prevent constructor conversion of uint256 to uint128 - make intent clear
* Prevent implicit conversion of uint64_t to uint256
* Prevent implicit conversion of uint64_t to uint160
Update README.md
Small Ledger cleanups and improvements:
 * Remove call to updateHash in LedgerConsensus::accept
 * Correct serialization size in updateHash
 * Add some comments to setImmutable and setAccepted
Optimize composition of HTTP/RPC reply strings
If RocksDB high threads are configured, use them for flushes
Allow range-based for to work on STVector256
Finish implementation of Ledger process/Features logic:
* Defining RIPPLE_PROPOSE_FEATURES enables ledger features
* Reduce dependence on getApp via dependency injection
* Converted uint32 members to use Clock::time_point
* Inject Journal, tidy up classes:
    - Features
    - FeaturesImpl
    - FeeVote
    - FeeVoteImpl