CommentManagerInterface for common functions to manage comment fields

A CommentManager service (id comment.manager) was added to allow a swappable back-end to comment module functionality. Its methods defined by the CommentManagerInterface so other services should inject it by referencing to this one.

Currently it implements following helper methods

getFields($entity_type) - returns an array of comment fields of the $entity_type

getAllFields() - returns all active comment fields

addDefaultField($entity_type, $bundle, $field_name = 'comment', $default_value = COMMENT_OPEN); - Attaches a comment field named 'comment' to the given entity type and bundle.

addBodyField($entity_type, $field_name) - Creates a comment_body field instance for the comment bundle defined by $entity_type and its $field_name the _comment_body_field_create() is gone in favor of it.