Fix uninitialized variable.

Per Coverity 1234603.
Fix missing release note entries in the previous release.
Fix missing release note entries in the previous release.
Fix to disable debug mode with a config reload

Per bug #114.
Fix possible segfault in query cache.

In ReadyForQuery, it does not check if session_context->query_context
exists. Thus it could lead to segfault later on if it does not exists.
Fix node id range check bug in trigger_failover_command().

The node id should be lower than NUM_BACKENDS. Probably harmless since
callers never pass node ids greater or equal to NUM_BACKENDS. But a
bug is a bug.