Logging api

Reviewers:  @bernerdschaefer, @peterbourgon 

First pass at the internals of the logging API.  This is two layers thick.  There is a RingBuffer for log messages, and on top of this is a data structure that implements storage for and operations for each container.  (The RingBuffer probably should have been a separate submit.)

The next piece to be worked on will be the HTTP logging interface.
Hmm. My feeling is that it's better to make that context switch explicit with a `time.Sleep` or `runtime.Gosched` in the appropriate place. But, YMMV.