common: add osd_distrust_data_digest option

common: add osd_distrust_data_digest option

If we have reason to distrust the stored full-object oi data digests,
this option will ignore them (and opportunistically clear them).  It
basically affects the same behaviors that osd_skip_data_digest except
the previous option only takes effect if bluestore is in use, while this
option is unconditional.

This serves a workaround for clusters that suffered the 12.2.6 bug that
produced bad full-object digests and *also* had PGs' primaries move from
bluestore to filestore nodes.  In those situations, this option can be
set to true.

After all OSDs are upgrade and all PGs come back with a clean deep
scrub this option can be disabled again.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit b8476ebc5b3c5b5d801bf875bbd6dce5b113325c)

# Conflicts:
#	src/osd/PrimaryLogPG.cc

- osd->osd_skip_data_digest, not g_conf->osd_skip_data_digest