This change causes each instance of Env to construct its own
isolated Application object for testing. Also included is
part of a framework to create multiple Application objects
in the same unit test and connect them together.
* Remove ripple::RippleMutex and ripple::RippleRecursiveMutex
and use std::mutex and std::recursive_mutex respectively.
* Use std::lock_guard instead of std::unique_lock when the
additional features of std::unique_lock are not needed.
