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)
