DFS block cache: fix lock issue and support parallel index loading

DFS block cache: fix lock issue and support parallel index loading

This change is a fix to http://git.eclipse.org/r/c/jgit/jgit/+/183562
that was reverted in http://git.eclipse.org/r/c/jgit/jgit/+/184978
due to deadlocks. Separate locks in DfsBlockFile are removed to rely
on getting value from DfsBlockCache with region locking in place.

With this change bitmap index creation is not blocked on index and
reverse index full initialization in DfsPackFile. Now bitmap index
and index could be read from storage in parallel in separate threads.

A unit test is added for parallel index loading.

Signed-off-by: Alina Djamankulova <adjama@google.com>
Change-Id: Ic6d9c5a4a254628636aa98a5008447a27a003f69