Bring Field API Memory improvements from Drupal 7.22 into Message module.

There's some new field api caching done with Drupal 7.22. In order to fully benefit from it, contrib modules need to update their code a little.

Here's the issue:
[#1915646]

Here's the types of code we need to update:

Iterate on all fields of an entity or a bundle
A. The typical code pattern for this in earlier Drupal 7 releases was :

// Get all field definitions in advance to save individual function calls to field_info_field().
$fields = field_info_fields();
foreach (field_info_instances($entity_type, $bundle) as $field_name => $instance) {
  $field = $fields[$field_name];
  // Do something with the $field and $instance
}

B. The memory-efficient way after Drupal 7.22 :

foreach (field_info_instances($entity_type, $bundle) as $field_name => $instance) {
  $field = field_info_field($field_name);
  // Do something with the $field and $instance
}

Iterate on all instances of a text field across all entity types and bundles
A. The typical code pattern for this in earlier Drupal 7 releases was :

foreach (field_info_instances() as $field_name => $instance) {
  $field = $fields[$field_name];
  if ($field['type'] == 'text') {
    // Do something with the $field and $instance
  }
}

B. The memory-efficient way after Drupal 7.22 :

foreach (field_info_field_map() as $field_name => $data) {
  if ($data['type'] == 'text') {
    $field = $fields[$field_name];
    foreach ($data['bundles'] as $entity_type => $bundles) {
      foreach ($bundles as $bundle) {
        $instance = field_info_instance($entity_type, $field_name, $bundle);
        // Do something with the $field and $instance
      }
    }
  }
}

NOTE: field_info_field_map() was added in Drupal 7.22, so contributed D7 modules should only use it within a check on the VERSION constant. Need to test to see if you can use it via

if (VERSION >= '7.22') {
  // snippet B.
}
else {
  // snippet A.
}

There's some places in message which might be able to benefit from a re-factor. I'll let the experts decided, but posting this issue to bring up the potential improvements.