Add base class for migrating reference fields

Problem/Motivation

We need to support migration of several different types of legacy reference fields to D8:

• Node and user reference, for both D6 and D7
• D7 taxonomy term reference fields
• D7 entity reference fields

Proposed resolution

Create a base class for migrate field plugins that handle all reference-type fields, since they tend to be more similar than they are different. Implement the node and user reference migrations for Drupal 6.

Remaining tasks

Write the patch, tests, docs. Let's pull ReferenceBase out of the patchzilla in #191, clean it up, and turn it into its own patch.

User interface changes

None.

API changes

• The namespace of the internal nodereference MigrateField plugin changes to Drupal\migrate_drupal\Plugin\migrate\field\d6.
• The internal nodereference MigrateField plugin now extends the new Drupal\migrate_drupal\Plugin\migrate\field\ReferenceBase class.

Release notes snippet

TBD