Change BlockCache from a static singleton to one per BlockManager

It would be useful to change BlockCache from being a static singleton to being instantiated and owned by a single FsManager or BlockManager. Right now we have a "hack" in LogBlockManager to randomize starting block IDs when we are running under GTest to avoid cache incoherency in between test runs. This causes differences in block id naming between the test environment and production, making it hard to catch certain types of problems.