Remove/reduce variable_set usage in hook_init

In hook_init() the module calls the below function which does a variable_set for the blazy library installation status.

function lazy_is_library_installed() {
  $blazy = libraries_detect('blazy');
  variable_set('lazy_library_installed', $blazy['installed']);

  return $blazy['installed'];
}

variable_set writes data into the 'variable' table, and clears the entire variable cache on every page request.

At the very least, there should be a check to see if the current value of the variable matches the blazy library status, avoiding the need for a variable update.