Refactor jtx::Env:

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)

src/ripple/unity/test.cpp

@@ -48,3 +48,4 @@
 #include <ripple/test/mao/impl/Net.cpp>
 #include <ripple/test/mao/impl/Net_test.cpp>
 
+#include <ripple/test/impl/ManualTimeKeeper.cpp>