Cell interrupts do not occur in background

Whenever Toree is run in the background - either directly from the shell or indirectly from the Jupyter stack that is started in the background - cell interrupts (via ctrl-C or SIGINT from parent) are not received - resulting in the inability to interrupt long-running cells.

This can be most simply demonstrated by invoking run.sh into the background (e.g., run.sh &) then issue ctrl-C (or `kill -2 <pid>`) to no avail.

This is related to TOREE-33 but only pertains to cell interrupt functionality since complete life-cycle management is assumed by the parent for the shutdown (double ctrl-C) scenario.