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Wrap slow query in PoolCounterWorkViaCallback

Description

Wrap slow query in PoolCounterWorkViaCallback

PoolCounterWorkViaCallback is available since MW 1.22. Also removed
the possibility to use action=purge. The cache times are already low:
one hour. Kept the member variable since it is handy to set it false
while developing.

Bug: T54728
Change-Id: Ib1b31327525c763b593eb71138b2cb9a1b41488c