[PATCH] CTDB recovery lock improvements

[Martin Schwenke]

Several improvements to the CTDB recovery lock and cluster mutex code:

* Remove "ctdb setreclock" command and supporting protocol/other code

  This can't be safely updated at run-time.  If it fails on a node
  then you probably need to take CTDB down on all nodes to be
  completely sure that you can get back into a consistent state.  So,
  even though this exists to avoid an outage, you would need to
  schedule a service window anyway, just in case!

  For those who think they really want to be able to update the
  recovery lock at run-time, set CTDB_RECOVERY_LOCK to a script and
  modify the script at run-time... but don't complain if bad things
  happen.  :-)

  Note that this came up when Amitay and I were discussing the
  introduction of a cluster lock, which is always held by the master
  and has nothing to do with recovery.  Should we add the ability to
  update the cluster lock?  No, as above...

  This is the first 10 commits.

* Cluster mutex improvements/simplifications:

  - Pass a talloc context to allocate the handle off, instead of
    always allocating off CTDB context

  - ctdb_cluster_mutex() register handler and private data

  - ctdb_cluster_mutex() register extra handler (and corresponding
    private data) to call when cluster mutex helper terminates
    unexpectedly.

* Allocate recovery lock handle off recovery daemon context

  CTDB context doesn't need this, since the lock is only ever taken in
  the recovery daemon.

* Add a SIGTERM handler to the recovery daemon that releases the
  recovery lock

  Before the recovery lock was handled by a helper, it was held by the
  recovery daemon itself.  Now, a helper can take a few seconds to
  notice the recovery daemon has exited, before it releases the lock
  and exits.  Main ctdbd shuts down recovery daemon with SIGTERM, so
  catch it, release lock, exit.

Please review and maybe push...

peace & happiness,
martin
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