OSD: pass param by ref to avoid copying overrhead

passing OpRequestRef, OSDMapRef by ref to avoid refcounting overhead
and in some cases passing structures by const ref.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
OSDMap: avoid passing pg_t by value

Passing pg_t param by cons ref for avoiding copies.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
PG: pass OpRequestRef by ref to avoid refcounting overhead

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
PG::op_mst_wait_for_map: pass the epoch rather than the map

have_same_or_newer_map is also modified to accept epoch instead of
map.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>