Investigate what cases could lead to an announcement signing loop

[ddustin]

There appears to be a case where nodes send announcement signatures back and forth in a tight loop. This could cause node symptoms like high CPU usage and perhaps high bandwidth usage.

A log of the tight loop is stored here:
https://gist.github.com/ddustin/dca9b0d1d0db7fe580661f493a059373

The log is from a PR and may simply be a side effect of that PR.