PG,ReplicatedPG: Generalize missing_loc for ECBackend

PG,ReplicatedPG: Generalize missing_loc for ECBackend

Prior to EC pools, unfound => missing.  Now, unfound (unreadable,
really) is dependent on the PGBackend requirements for reconstituting
an object.  This also means recovering an object missing on a replica
but not the primary requires tracking the missing_loc set.

Thus, rather than maintaining missing_loc only for objects missing
on the primary, the MissingLoc structure will track all missing
objects actingbackfill-wide until each object is recovered.

For simplicity, since we don't really know what objects need recovery
until activation (and since we can't do anything with that information
prior to activation anyway), we defer populating the missing_loc
information until activation.

We need peers to rollback divergent log entries before we attempt to
read the relevant objects.  The simplest way to accomplish this seems to
be the simply choose to always activate peers if search_for_missing
turns up missing objects.

Due to EC pools, missing is necessary, but not sufficient for readability.
Thus, we instead check is_unreadable for cases where we need to read the object
and reserve is_missing for cases where we need the object context.

wait_for_missing_object becomes waiting_for_unreadable_object in order to avoid
having another layer of waiting_for_* maps.  These ops may be requeued
either when the primary is recovered or when the object is no longer degraded,
depending on when the object becomes readable.

Signed-off-by: Samuel Just <sam.just@inktank.com>