SI-7115  JMapWrapper.get can incorrectly return Some(null)

This isn't incorrect.  Trying to use a single-threaded interface in a concurrent context is supposed to break in various unpleasant ways.

Documentation has been added to encourage one to avoid wrapping a concurrent map in the generic wrapper (which assumes a single thread), and pointing out that synchronized maps do not maintain synchronization for non-atomic operations (including get).

More docs.
Merge pull request #3839 from Ichoran/issue/7115

SI-7115  JMapWrapper.get can incorrectly return Some(null)