Previously, child events could be published before their parent events during a race in the `BuildEventStreamer#post` method. This method was not synchronized around delivering events to the `BuildEventTransport` objects, so it was possible for this race to occur:

Thread #1 | Thread #2
----------|-----------
make parent event |   
post parent event |   
  | make child event
  | post child event
  | deliver child event
deliver parent event |  

TESTED: extensive benchmarking rules out increase in wall time or heap high-water-mark.

RELNOTES: BEP events will now be written respecting parent-child ordering constraints.
PiperOrigin-RevId: 884592559
Change-Id: Iaaac7f48b849c146632fbc10ce3c2e4e1fa6af07