osd: Generalise can_serve_replica_read for consumption by EC.

osd: Generalise can_serve_replica_read for consumption by EC.

The can_serve_replica_read() function is called by replica to determine whether there are
any uncommitted writes.  If such writes exist, then the system will reject the IO to avoid
the risk of reading data from a write which may yet be rolled back.

The same code is going to be useful for EC direct reads.

The string_view code is not expensive.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>