Allow to listen Unix socket (#2085)

* Allow puma to listen Unix socket

* streaming: Show the whole listening address instead of the port

Port is not always appropriate (e.g. Unix socket)

* streaming: Close server before exiting

This change especially allows to remove Unix socket before exiting.

config/puma.rb

@@ -1,6 +1,10 @@
 threads_count = ENV.fetch('MAX_THREADS') { 5 }.to_i
 threads threads_count, threads_count
 
+if ENV['SOCKET'] then
+  bind 'unix://' + ENV['SOCKET']
+end
+
 port        ENV.fetch('PORT') { 3000 }
 environment ENV.fetch('RAILS_ENV') { 'development' }
 workers     ENV.fetch('WEB_CONCURRENCY') { 2 }