Propagate commit and revert metadata event to other nodes in cluster.

Propagate commit and revert metadata event to other nodes in cluster.

This patch tries to correctly track changes in lvmcache related to commit/revert.

For vg_commit: if there is cached precommitted metadata, after successfull commit
these metadata must be tracked as committed.

For vg_revert: remote nodes must drop precommitted metadata and its flag in lvmcache.

(N.B. Patch do not touch LV locks here in any way.)

All this machinery is needed to properly solve remote node cache invalidaton which
cause several problems recently observed.