add Unicorn::OobGC middleware

add Unicorn::OobGC middleware

This middleware allows configurable out-of-band garbage
collection outside of the normal request/response cycle.

It offers configurable paths (to only GC on expensive actions)
and intervals to limit GC frequency.

It is only expected to work well with Unicorn, as it would
hurt performance on single-threaded servers if they
have keepalive enabled.  Obviously this does not work well
for multi-threaded or evented servers that serve multiple
clients at once.