Add the delete() operation to SharedLock

This new operation lets a lock be cleanly deleted. The lock will be exclusively
held before deletion, and after it pending and future acquires will raise an
exception. Other SharedLock operations are modify to deal with delete() and to
avoid code duplication.

This patch also adds unit testing for the new function and its interaction with
the other lock features. The helper threads are sligtly modified to handle and
report the condition of a deleted lock. As a bonus a non-related unit test
about not supporting non-blocking mode yet has been added as well.

This feature will be used by the LockSet in order to support deadlock-free
delete of resources. This in turn will be useful to gracefully handle the
removal of instances and nodes from the cluster dealing with the fact that
other operations may be pending on them.

Reviewed-by: iustinp