ACCUMULO-2695 Fixed Conditional writer hang caused by tablet server fault

This change fixes issues that occurred when the conditional writer got an
exception when trying to connect to a tserver.  The conditonal writer could
throw an exception or loose mutations in this case.  In either case the client
would hang.
Merge branch '1.6.0-SNAPSHOT'
ACCUMULO-2686 Label classes with correct @since tag
Merge branch '1.6.0-SNAPSHOT'