Enforce cache size on read requests

In-flight cache reads were not previously counted against
new cache read requests, which could result in very large
cache usage.  This effect is most noticeable when writing
small chunks to a cloned image since each write requires
a full object read from the parent.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 4fc9fffc494abedac0a9b1ce44706343f18466f1)
rbd: ObjectCacher reads can hang when reading sparse files

The pending read list was not properly flushed when empty objects
were read from a space file.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit cdb7675a21c9107e3596c90c2b1598def3c6899f)