SCHEMA-749. Fix memory leak in layout watching.

When we create a layout tracker, there are some series of references where one
of them is a watcher that gets passed to zookeeper (a long lived thread). This
watcher has a reference (since it's a non-static inner) to its parent object
which has a non-null reference to the layout tracker which is effectively the
HBaseKijiTable. In a multiple classloader environment, this prevents the parent
classloader from being unloaded and hence a memory leak.

By nulling out the layout tracker reference on close, we break this dependency
only leaving a "lightweight" reference to the watcher. Until we can remove
watchers, this will continue to be the case.

Review: https://review.kiji.org/r/1789/
Reviewer: milo