Replaced lock for ReaderWriterLockSlim in dvcs graph

I replaced lock in DvcsGraph for ReaderWriterSlimLock. Also added extensions methods to make it possible to use ReaderWriterSlimLock in using block.

I have a very large repository with 1000+ branches and 100.000+ commits. This patch makes scrolling in the revision graph more responsive for me.

Can someone review this change?