Gracefully shutdown HTTP, gRPC, and Prom servers

[haydentherapper]

This catches sigint and sigterm and calls each server's shutdown method, blocking until shutdown completes.

This is only added for non-Duplex, we'll need to add a shutdown method for Duplex servers.

Fixes #1341

Summary

Release Note

Documentation

[haydentherapper]

Tested with go run main.go serve --port 5555 --grpc-port 5554 --ca ephemeralca --ct-log-url="" and confirmed we saw all expected logging statements.

I tried to get this working for Duplex by calling d.Server.GracefulStop(), but the process hung. This is because Serve() doesn't expose the server to stop. Will need to update Duplex, but we don't use this in prod.

[codecov]

Codecov Report

Merging #1342 (cefc964) into main (f94df0a) will decrease coverage by 0.93%.
Report is 9 commits behind head on main.
The diff coverage is 30.43%.

@@            Coverage Diff             @@
##             main    #1342      +/-   ##
==========================================
- Coverage   58.49%   57.56%   -0.93%     
==========================================
  Files          50       50              
  Lines        3053     3111      +58     
==========================================
+ Hits         1786     1791       +5     
- Misses       1110     1161      +51     
- Partials      157      159       +2     

Files Changed	Coverage Δ
cmd/app/http.go	48.93% <0.00%> (-11.60%)	⬇️
cmd/app/serve.go	25.47% <3.57%> (-1.77%)	⬇️
cmd/app/grpc.go	74.72% <90.90%> (-1.79%)	⬇️

... and 1 file with indirect coverage changes

[cpanato]

Cool

[bobcallaway]

if ports are already in use (e.g. tcp :2112: bind: address already in use), the server will hang instead of quickly failing and exiting

[haydentherapper]

@bobcallaway Fixed comments, also made the unix domain socket configurable so that you can start up two fulcios on the same machine