FLUME-1947. ExecSource should optionally block if the channel is full.
Review Request #9914 - Created March 13, 2013 and updated
Made multiple changes to the exec source: 1. Flush the events to the channel, based on a configurable timeout - even when a full batch was not received. 2. Block the spawned process until channel exceptions are cleared up by not reading the stdout buffer. Backoff exponentially (subject to an upper limit), till events are available.
Added unit tests for the new features. All current tests pass.