DEV: ensure we never fork v8 contexts from unicorn

v8 forking is not supported and can lead to memory leaks.

This commit handles the most common case which is the unicorn master forking

There are still some cases related to backup where we fork, however those
forks are usually short lived so the memory leak is not severe, burning
the contexts in the master process could break sidekiq or web process that
do the actual forking

config/unicorn.conf.rb

@@ -69,6 +69,9 @@ before_fork do |server, worker|
     Discourse.git_branch
     Discourse.full_version
 
+    # V8 does not support forking, make sure all contexts are disposed
+    ObjectSpace.each_object(MiniRacer::Context) { |c| c.dispose }
+
     # get rid of rubbish so we don't share it
     GC.start