osd/PGLog: write only changed dup entries

osd/PGLog: write only changed dup entries

Writing all of the dup entries whenever one is changed causes a large
regression in performance.

Instead, keep track of ranges that need to be cleared and written
after log merging (dirty_{to,from}_dups) and the earliest dup entry we
haven't written yet during normal operation (write_from_dups). This
parallels the way we track unpersisted log entries.

Check that the correct set of dups is persisted by adding a roundtrip
through memstore to each merge_log unit test.

Fixes: http://tracker.ceph.com/issues/21026
Signed-off-by: Josh Durgin <jdurgin@redhat.com>