Add support for live builds

[EdSchouten]

Right now bb-event service saves up all events for a build and only flushes it into the CAS/AC upon completion of the build. We should add support for live viewing of builds that are in progress.

An interesting problem in this area is that we preferably should not write a full event log into the CAS for every tiny update. We should likely add some delay to write out multiple events in one go. We are allowed to split up the event log across multiple files in the CAS, but splitting it up in too many chunks will degrade performance as well. Maybe some trick exists that doesn't waste too much space, but limits output to O(log n) blobs?