Fix display formatting of STAmount
RPC: Improve wallet_propose, deprecate wallet_seed:
* Add master_key, public_key and public_key_hex to wallet_propose's JSON response.
* Report an error if we can't understand wallet_propose's passphrase.
* Deprecate wallet_seed by adding a "deprecated" parameter directing the user to wallet_propose.
If RocksDB high threads are configured, use them for flushes
[WIP] Features/IFeatures cleanups
Allow range-based for to work on STVector256
[WIP] Complete change process / Features logic
Refactor, tidying for class Features:
* Inject Journal, tidy up classes:
    - Features
    - FeaturesImpl
    - FeeVote
    - FeeVoteImpl
* RIPPLE_NO_PROPOSE_FEATURES deactivates ledger features
* Reduce dependence on getApp via dependency injection