crimson/osd/: restructure client and peering operation handling

crimson/osd/: restructure client and peering operation handling

This patch:
* Breaks pg map handling out into a separate module (pg_map.*)
* Breaks osdmap waiting into a separate module
  - Ops actually need to wait twice (potentially): once at
    the osd and a second time at the pg.  The same structure
    is used for both.
* Op ordering is enforced via OrderedExclusivePipelineStages
  defined in osd_operations.h.
  - Peering and client ops each define a pipeline section
    centered on the connection as well as one centered on the
    pg.

Signed-off-by: Samuel Just <sjust@redhat.com>