Add Log Block Manager support for multiple un-synced blocks in a container

Per adar, the Log Block Manager treats new blocks as either being open (and thus not fsync'ed) or closed (and thus fsync'ed), but an open block is also considered to be in progress, thus preventing its container from being used for other new blocks.

If we want to support bulk fsync after many tablet operations, such as what we would like to do in Tablet Copy, then we need to take this apart; otherwise deferring fsyncs for 1000 blocks means 1000 open containers.