osd_types: switch pg_pool_t cache-related fields to be uint64_t

The pool names can be renamed; we need to refer to them by their
authoritative IDs. In addition to that, switch from a single "overlay"
to a "read_tier" and "write_tier". These two flags are all the clients
need to look at when generating requests to support basic caching
and tiering.

Signed-off-by: Greg Farnum <greg@inktank.com>
OSDMonitor: switch 'osd pool tier' manipulation code to work with int64_t IDs

Signed-off-by: Greg Farnum <greg@inktank.com>
OSDMonitor: use get_cache_mode_name() instead of returning cache mode ID

Signed-off-by: Greg Farnum <greg@inktank.com>