* Test whether offers which either already below the reserve (or
would go below during processing) can execute if they cross.
* Test the "Fill or Kill" and "Immediate or Cancel" flags.
These changes eliminate the Env's OpenLedger member and make
transactions go through the Application associated with each
instance of the Env, making the unit tests follow a code path
closer to the production code path.
* Add Env::open() for open ledger
* Add Env::now()
* Rename to Env::current()
* Inject ManualTimeKeeper in Env Application
* Make Config mutable
* Move setupConfigForUnitTests
* Launch Env Application thread
* Use Application ledgers in Env
* Adjust Application clock on ledger close
* Adjust close time for close resolution
* Scrub obsolete clock types
* Enable features via Env ctor
* Make Env::master Account object global
* Cache SSL context (performance)
* Cache master wallet keys in Ledger ctor (performance)
Very small payment could fail when STAmount::mulRound underflowed
and returned zero, when it should have rounded up to the smallest
representable value.
