mon/PGMap: add unit test for min_last_epoch_clean

Signed-off-by: Sage Weil <sage@inktank.com>
mon/PGMap: fix osd_epochs update

The insert() call here does not overwrite a previous entry, which means
that the osd_epochs map is never moving forward in time.  This seems to
have been broken since it was introduced in 091809b814.

Backport: emperor, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1304 from ceph/wip-last-epoch-clean

mon: fix min_last_epoch_clean handling

Reviewed-by: Samuel Just <sam.just@inktank.com>