Wait 5 seconds instead of 2 seconds to read stderr from launched server
Flush output every time we write to it in StreamDumper

To try harder to get our errors displayed.
Don't delete the server config file if SBT_SERVER_SAVE_TEMPS ne null

This allows people to reproduce failures by running the command line
by hand.

Fixes #1394
Merge pull request #1397 from havocp/wip/havocp-try-harder-for-errors

Try harder to get diagnostics on server launch failure