LU-12553 mdc: polling mode for changelog reader

LU-12553  mdc: polling mode for changelog reader

this allows the user (like lsom_sync and similar) to follow
the changelog and don't rescan getting duplicates.

Lustre-change: https://review.whamcloud.com/35262
Lustre-commit: e215002883d5620f43615013452935da8e7e3f8c

Change-Id: I78dc163838c1b88f9447a4731ad4bfe00fec7eff
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36362
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>