Enforce cache size on read requests

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)

Conflicts:
	src/osdc/ObjectCacher.h