libuv plugin does not track real work vs idle work.

The libuv plugin for libcouchbase does not properly keep track of 'active work' vs 'idle work' when dispatching libuv watchers. This causes an issue where the libuv event loop will continue idling even though there is no real work to complete. This particularly affects the node.js driver as a script that no longer has any work left will never exit if a Couchbase connection is still open (even though the connection is idle).