osd: simplify pg write mode transitions

osd: simplify pg write mode transitions

There is no reason to track client on the per-pg AccessMode, since we only
care about read/write races on a per-object state, and need to track that
per-object anyway due to the new async commit/apply model.  (Though this
is semi-broken even without that...)

Currently we will move from delayed to rmw, but there is no rmw->delayed
transition, since the logic for that is a bit more complex.. it'll be
difficult to determine when it is really a win.

In general, though, RMW is probably a better all around policy!