Can't exit to SBT prompt if kernel running

[chrismyang]

1. Run SN from sbt (i.e., "project server; run")
2. Don't open a notebook! Simply type any key: you get returned to the SBT prompt
3. Now open a notebook (make sure the kernel comes up)
4. Hit a key.

Expected: returned to SBT prompt Actual: Stuck in SBT netherworld, an unceasing nightmare where you are forced to either exit the SBT process entirely, or wait an eternity for a sweet release that will never arrive.

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We're getting out the the Server main method, so I'm like 99% sure that there's just some daemon thread that SBT is blocking on to complete.

(In case it's helpful, the remote actor child process is definitely exiting).