mds: go back to compact_map for replicas

mds: go back to compact_map for replicas

Zheng observed that an alloc_ptr doesn't really work in this case since any
call to get_replicas() will cause the map to be allocated, nullifying the
benefit. Use a compact_map until a better solution can be written. (This means
that the map will be allocated outside the mempool.)

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>