Improve file lock contention handling This commit improves the likelihood for lock requesters to acquire a file lock after it is released due to contention. After the lock has been released, the former lock holder now sends a packet to the sockets of all requesters. While old clients will simply ignore the additional packet, new clients will interpret it as a signal that the file lock has been released and will try to acquire it immediately.
Provide an empty contended action instead of null This contention handling is already active for exclusive locks, but without an action it is ignoring requests from other parties. With this, it sends confirmation that the request was received and that lock releasing is in progress. Although the release itself is not performed by the contended action, but manually after the lock is no longer needed (which is the contract for exclusive locks).