CLEANUP: pools: uninline pool_put_to_cache()

CLEANUP: pools: uninline pool_put_to_cache()

This function has become too big (251 bytes) and is now hurting
performance a lot, with up to 4% request rate being lost over the last
pool changes. Let's move it to pool.c as a regular function. Other
attempts were made to cut it in half but it's still inefficient. Doing
this results in saving ~90kB of object code, and even 112kB since the
pool changes, with code that is even slightly faster!

Conversely, pool_get_from_cache(), which remains half of this size, is
still faster inlined, likely in part due to the immediate use of the
returned pointer afterwards.