[MEDIUM] ensure we don't recursively call pool_gc2()

[MEDIUM] ensure we don't recursively call pool_gc2()

A race condition exists in the hot reconfiguration code. It is
theorically possible that the second signal is sent during a free()
in the first list, which can cause crashes or freezes (the later
have been observed). Just set up a counter to ensure we do not
recurse.
(cherry picked from commit b7f9d126e269f3b5b7dc05e39fcf207ba86a330c)
(cherry picked from commit 5a01de1c74b8c1c0c2a0e62e9fb6b432e24c79f9)