OSDMap: deepish_copy_from()

Make a deep(ish) copy of another OSDMap.  Unfortunatley we can't make the
compiler-generated copy operator/constructors private until c++11.  :(

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit bd54b9841b9255406e56cdc7269bddb419453304)
OSDMonitor: use deepish_copy_from for remove_down_pg_temp

This is a backport of 368852f6c0a884b8fdc80a5cd6f9ab72e814d412.

Make a deep copy of the OSDMap to avoid clobbering the in-memory copy with
the call to apply_incremental.

Fixes: #7060
Signed-off-by: Sage Weil <sage@inktank.com>