Broken boolean values

The way it is now, doing a cache.add has a slight race condition
between checking if the key exists already before setting.

This rewrite uses the native SETNX command to only set the key if one
doesn't exist yet.

Adding test to prove multiple connections are being formed when accessing multiple backends.  Thanks to @twidi for the report and jezdez for the kick in the pants.

Modified the timeout value semantics to better match the memcache cache backend, i.e. negative timeouts are instant expiration, 0 timeout is no expiration, and greater than 0 is time to live.