Improved log support, handle deprecations and added dump() function

• Added support for Symfony dump() method for dumping variables.
  It can be used instead of var_dump() as it has better output
  and output will also be present for error pages.
• Env variable STARTER_BOOTSTRAP_MODE now also controls
  STARTER_FRAMEWORK unless it is set. This makes it possible to
  start plain mode by setting $GLOBALS["STARTER_FRAMEWORK"]="plain";
• eZ publish legacy kernel can be found using a bootstrap file called
  vendor/ezsystems/ezlegacy/bootstrap_kernel.php.
  This will be executed and may set the EZP_ROOT env variable.
• Fixed bug in BaseApp which prevented log processors to be attached to loggers.
• Added support for configuring log formatters and assigning them to log handlers.
• Added support for setting up a log introspector, this will attach
  file and line numbers to log messages. The introspector can be
  enabled by setting env variable LOG_INTROSPECT=1.
• Default log function starter_log* now uses app.defaultLog to
  determine which log channel to use. Use function
  starter_log_name to get the name of the default channel.
• Added prevention of recursive calls to fetchLoggers().
• Added flag app.deprecation to control how deprecation errors are
  treated. They can either be treated as an error, logged as a
  an error or ignored. Defaults to logging for prod and error for
  development.
  This can be controlled at run-time by setting the env variable
  ERROR_DEPRECATION to one of error, log or ignore.