Implement back pressure

logtron had a bug in the way it wrote to it's streams
    by not respecting back pressure.

This code adds:

 - The ability to configure a `highWaterMark` for all
    the streams created by `logtron`.
 - Back pressure. If a stream is full we drop information.
 - Test for back pressure, there is a detailed test that
    asserts that we do not blow up in memory.

cc @sh1mmer @kriskowal
Can you explain why you changes this?