HTable#put improvements

3 points:

• When doing a single put, we're creating an object by calling Arrays.asList
• we're doing a size check every 10 put. Not doing it seems simpler, better and allows to share some code between a single put and a list of puts.
• we could call flushCommits on empty write buffer, especially for someone using a lot of HTable instead of using a pool, as it's called in close().