Add a quota attach function with a callback, some code cleanups.

We introduce a isc_quota_attach_cb function - if ISC_R_QUOTA is returned at the time the function is called, then a callback will be called when there's quota available (with quota already attached). The callbacks are organized as a LIFO queue in the quota structure. It's needed for TCP client quota - with old networking code we had one single place where tcp clients quota was processed so we could resume accepting when the we had spare slots, but it's gone with netmgr - now we need to notify the listener/accepter that there's quota available so that it can resume accepting.

Remove unused isc_quota_force() function.

The isc_quote_reserve and isc_quota_release were used only internally from the quota.c and the tests. We should not expose API we are not using.

(cherry picked from commit d151a10f)