= Improvements

* Running migrations using one of the included migrators on separate
  Database objects in separate threads simultaneously is now
  supported.  Previously, the migrators were not thread-safe.

* On Ruby 2.5+, :db_type entries in the schema hashes are now deduped
  for a slight memory savings when using many columns with the same
  database type.

* The schema_caching extension now freezes string values in the
  resulting hashes, just as the default schema parsing code started
  doing in 5.5.0.

* The schema_caching extension now supports the :callable_default
  schema values used by the pg_json, pg_array, and pg_hstore
  extensions, by removing the entry before caching and resetting it
  after restoring the cache.

* Identifier mangling rules are now respected when renaming columns on
  Microsoft SQL Server.

= Backwards Compatibility

* The migrator internals were modified in order to support
  thread-safety.  The private Migrator#remove_migration_classes
  method has been removed, and #load_migration_file now returns the
  migration object/class instead of populating Migration.descendants.
  Migration.descendants is now only used for temporary storage, and
  will no longer contain all migration objects/classes used by the
  migrator.