Add new port databases/pecl-yac

PR: 194553
Submitted by: John Pupu

Yac is a shared memory user data cache for PHP. It can be used instead of
APC or local memcached.

Yac is lockless, which it is very fast, but there could be a chance the
wrong data will be retrieved (it depends on how many key slots are
allocated and how many keys are stored), so a product using Yac should not
be very sensitive to data loss.