MINOR: pools: do not maintain the lock during pool_flush()

MINOR: pools: do not maintain the lock during pool_flush()

The locked version of pool_flush() is absurd, it locks the pool for each
and every element to be released till the end. Not only this is extremely
inefficient, but it may even never finish if other threads spend their
time refilling the pool. The only case where this can happen is during
soft-stop so the risk remains limited, but it should be addressed.

(cherry picked from commit c88914379da35c46d093f6d410b9507355aacd0a)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit af8120a53cf6a7a8e244135893195c6b65ccc91a)
[wt: in 2.3 the pool_flush code is different because it still requires
 to manipulate ->allocated under the lock, thus the lock covers the
 counting and detaching, and the freeing is performed out of the lock]
Signed-off-by: Willy Tarreau <w@1wt.eu>