Allow hooks to be skipped with SKIP_HOOKS env variable

`SKIP_CHECKS` was the old form, and `SKIP` is the preferred new form,
but it is likely that new users will try `SKIP_HOOKS`, so add support
for it.

Change-Id: Ie03c62fc68366de42113282d07560af00e695acc
Parallelize hook runs

Add support for running hooks in parallel if the machine has multiple
processors. This drastically improves performance when running slower
hooks.

This required adding a mutex to the `HookRunner` in order to synchronize
access to STDOUT.

Change-Id: I400034a0d2d7a84d01efc37e517c30af4fe959e0