commands: add support for gat commands [#5]

New APIs omcache_gat and omcache_gat_multi for get-and-touch operations.
Note that the GATKQ command is somewhat broken in current memcached versions
and doesn't return the key: https://github.com/memcached/memcached/pull/85
test_servers: test literal ipv6 addresses

Commit 12a562a57 modified the ipv6 test to use symbolic names to test name
resolution with getaddrinfo.  That was the only case using [addr]:port style
addresses so the functionality wasn't tested at all anymore.  Updated
test_server_list to use that form, too.