configurable database pool sizes

lib/postal/config.rb

@@ -74,7 +74,7 @@ module Postal
 
   def self.database_url
     if config.main_db
-      "mysql2://#{CGI.escape(config.main_db.username.to_s)}:#{CGI.escape(config.main_db.password.to_s)}@#{config.main_db.host}:#{config.main_db.port}/#{config.main_db.database}?reconnect=true&encoding=#{config.main_db.encoding || 'utf8mb4'}"
+      "mysql2://#{CGI.escape(config.main_db.username.to_s)}:#{CGI.escape(config.main_db.password.to_s)}@#{config.main_db.host}:#{config.main_db.port}/#{config.main_db.database}?reconnect=true&encoding=#{config.main_db.encoding || 'utf8mb4'}&pool=#{config.main_db.pool_size}"
     else
       "mysql2://root@localhost/postal"
     end