More fixes for prompt logger

[lihaoyi]

1. Make EvaluatorCore waits for all non-exclusive tasks to finish before beginning to run exclusive tasks to prevent overlap
2. Re-organize promptLineState/streamManager/runningState to ensure each piece of mutable state is only read by one of them
3. Tweak PromptLoggerUtil.clearScreenToEndBytes to prevent the clearScreen from mucking up tab-stops in iTerm2
4. Overhaul PromptLogger's synchronization strategy: we now no longer synchronize on child loggers writing to PromptLogger's streams, and only synchronize on PromptLogger writing to the parent systemStreams0 (including the writes coming from ProxyStream.Pumper) along with the operations mutating state within PromptLogger
5. Ensure we only print the prompt in preRead if the previous character was a \n, to ensure that when log lines are broken over multiple read/writes we do not allow a prompt to be written in between the two parts of the line

(3) seems to fix #3752