Run commands in a separate executor instead of the main kernel thread

This fixes a bug where interrupting a command would terminate the main
event loop thread, causing the kernel to stop responding. This happened
because the interrupt handling sent a Java interrupt to the thread
executing the current command, which was always the main event loop
thread. Now commands are executed on a separate executor, which can
safely have Java interrupts sent to it without affecting any
jupyter-jvm-basekernel threads.