ObjectCacher: add a method to clear -ENOENT caching

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>