The only external use of Close was always followed by an implicit destruction
(by resetting the RefPtr), so we don't need to expose it, and it can be done
from the destructor.

FileBlockCache keeps its Close() function for internal use.
Also, FileBlockCache::mIsOpen is redundant, as it's true iff mThread is not
null.

MozReview-Commit-ID: LV7YVrwJvGG

--HG--
extra : rebase_source : 23decadf249b9e63190b3e19d81edc4a090afcef