ObjectCacher: keep track of outstanding reads on an object

Reads always use C_ReadFinish as a callback (and they are the only
user of this callback). Keep an xlist of these for each object, so
they can remove themselves as they finish. To prevent racing requests
and with discard removing objects from the cache, clear the xlist in
the object destructor, so if the Object is still valid the set_item
will still be on the list.

Make the ObjectCacher constructor take an Object* instead of the pool
and object id, which are derived from the Object* anyway.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
ObjectCacher: add a method to clear -ENOENT caching

Clear the exists and complete flags for any objects that have exists
set to false, and force any in-flight reads to retry if they get
-ENOENT instead of generating zeros.

This is useful for getting the cache into a consistent state for rbd
after an image has been flattened, since many objects which previously
did not exist and went up to the parent to retrieve data may now exist
in the child.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: invalidate cache when flattening

The cache stores which objects don't exist. Flatten bypasses the cache
when doing its copyups, so when it is done the -ENOENT from the cache
is treated as zeroes instead of 'need to read from parent'.

Clients that have the image open need to forgot about the cached
non-existent objects as well. Do this during ictx_refresh, while the
parent_lock is held exclusively so no new reads from the parent can
happen until the updated parent metadata is visible, so no new reads
from the parent will occur.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>