Fix the new_crashes generator for HBase so it doesn't close the connection too early.

Also, don't close the connection when requested for the persistent HBase
connection (and only close it when the context is closed).
Add a force_reconnect method to connection_contexts.

This allows us to reestablish persistent connections that may die.

Also add unit tests for the persistent HBase context.
Merge pull request #1113 from rfw/hbase_fix_new_crashes

Persistence on disconnect for HBase and fixed new_crashes generator