ApiServer should shutdown gracefully

[snape479]

Previously, 'with_graceful_shutdown' was added to allow the ApiServer to shutdown gracefully here:

4a09fed

However, it had to be reverted due to the fact that the 'rx' was being dropped at the end of the code block and a 'cancel' error occurred right away. The solution to this problem is to store the (tx, rx) on the heap using Box and moving them to the startup routines in src/bin/grin.rs. Additionally, a monitor thread is needed to call stop when the StopState indicates the server stopped.

I have implemented this in bitcoinmw here:

bitcoinmw@2f25ec2

I was going to do a pull request to Grin, but my repo had some merge issues (due to verifier_cache). If someone could copy these changes from BMW over to Grin that would be great. Opening this issue to track.

[phyro]

I believe this is now resolved. Thanks for the issue and the fix.