Summary:
Currently, the first thing you want to do when getting a context from the pool is
reset it, because you don't know the state the last user left it in. This is bug
prone, because you may forget to reset the context. Instead, the pool should
handle the resetting logic, so it cannot be forgotten.

This diff adds a `Resetter` to the pool, and the reset function is called every
time a context is returned to the pool. I chose to reset when returning to the
pool instead of when retrieving a context because that gives the context a
chance to free any resources it doesn't expect to need in the future before it
potentially becomres idle.

Reviewed By: Cyan4973

Differential Revision: D26439710

fbshipit-source-id: 8b9afa3db2fda4d167f0fd5307791e8a8a4f283c