Add support for unix socket of memcached
Add support for unix socket of memcached

python-memcached supports unix sockets with "unix:/path/to/socket". So the
cache_stats should support that scheme, too.

Testing Done:
Started memcached with:
  * memcached -l 127.0.0.1 -p 11211
  * memcached -s /tmp/socket

Tried path "unix:/tmp/socket" and "localhost:11211" as the memcached connection
in admin. "Cache Stats" in admin dashboard works correct and caching of diffs
in reviews, too.

Reviewed at https://reviews.reviewboard.org/r/7940/

brennie/reviewboard,davidt/reviewboard,sgallagher/reviewboard,brennie/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,davidt/reviewboard,davidt/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,brennie/reviewboard,brennie/reviewboard,davidt/reviewboard,sgallagher/reviewboard