DEV: Improve `bin/unicorn` boot time in development environment (#12900)

This commit makes a few changes to improve boot time in development environments. It will have no effect on production boot times.

- Skip the SchemaCache warmup. In development mode, the SchemaCache is refreshed every time there is a code change, so warmup is of limited use.

- Skip warming up PrettyText. This adds ~2s to each web worker's boot time. The vast majority of requests do not use PrettyText, so it is more efficient to defer its warmup until it's needed

- Skip the intentional 1 second pause during Unicorn worker forking. The comment (which also exists in Unicorn's documentation) suggests this works around a Unix signal handling bug, but I haven't been able to locate any more information. Skipping it in dev will significantly speed up boot. If we start to see issues, we can revert this change.

On my machine, this improves `/bin/unicorn` boot time from >10s to ~4s

config/unicorn.conf.rb

@@ -251,18 +251,19 @@ before_fork do |server, worker|
   # to the implementation of standard Unix signal handlers, this
   # helps (but does not completely) prevent identical, repeated signals
   # from being lost when the receiving process is busy.
-  sleep 1
+  sleep 1 if !Rails.env.development?
 end
 
 after_fork do |server, worker|
   DiscourseEvent.trigger(:web_fork_started)
+  Discourse.after_fork
 
   # warm up v8 after fork, that way we do not fork a v8 context
   # it may cause issues if bg threads in a v8 isolate randomly stop
   # working due to fork
-  Discourse.after_fork
   begin
-    PrettyText.cook("warm up **pretty text**")
+    # Skip warmup in development mode - it makes boot take ~2s longer
+    PrettyText.cook("warm up **pretty text**") if !Rails.env.development?
   rescue => e
     Rails.logger.error("Failed to warm up pretty text: #{e}")
   end

lib/discourse.rb

@@ -903,6 +903,10 @@ module Discourse
   def self.preload_rails!
     return if @preloaded_rails
 
+    if !Rails.env.development?
+      # Skipped in development because the schema cache gets reset on every code change anyway
+      # Better to rely on the filesystem-based db:schema:cache:dump
+
       # load up all models and schema
       (ActiveRecord::Base.connection.tables - %w[schema_migrations versions]).each do |table|
         table.classify.constantize.first rescue nil
@@ -912,6 +916,7 @@ module Discourse
       ActiveRecord::Base.connection.data_sources.each do |table|
         ActiveRecord::Base.connection.schema_cache.add(table)
       end
+    end
 
     schema_cache = ActiveRecord::Base.connection.schema_cache