PERF: avoid shelling to get hostname aggressively

Previously we had many places in the app that called `hostname` to get
hostname of a server. This commit replaces the pattern in 2 ways

1. We cache the result in `Discourse.os_hostname` so it is only ever called once

2. We prefer to use Socket.gethostname which avoids making a shell command

This improves performance as we are not spawning hostname processes throughout
the app lifetime

lib/discourse_logstash_logger.rb

@@ -8,7 +8,7 @@ class DiscourseLogstashLogger
       uri: uri,
       sync: true,
       customize_event: ->(event) {
-        event['hostname'] = `hostname`.chomp
+        event['hostname'] = Discourse.os_hostname
         event['severity_name'] = event['severity']
         event['severity'] = Object.const_get("Logger::Severity::#{event['severity']}")
         event['type'] = type