Resource group used to access resGroupSlot in SessionState without
lock. This is correct when session only access resGroupSlot by itself.
But as we introduced runaway feature, we need to traverse the current
session array to find the top consumer session when redzone is reached.
This requires:
1. runaway detector should hold shared resgroup lock to avoid resGroupSlot
is detached from a session concurrently when redzone is reached.
2. normal session should hold exclusive lock when modifying resGroupSlot
in SessionState.

Also fix a compile warning.
Reviewed-by: NNing Yu <nyu@pivotal.io>