Fix possible increasing in memory allocation if refreshing_context

Fix possible increasing in memory allocation if refreshing_context
(for example when CLVMD_CMD_LOCK_VG for is called during vgscan).

If clvmd calls LV lock, it calls
    /* clean the pool for another command */
	dm_pool_empty(cmd->mem);
to clean up memory pool after command.

Unfortunately, do_refresh_cache() do not call this
and because during it operation it allocates some memory,
the pool increases.

Also do_refresh_cache should use lvm_lock
(it manipulates with lvm internal data).

The same applies for lvm_backup command.

Signed-off-by: Milan Broz <mbroz@redhat.com>