Remove peer dependency on datasource-juggler

Use `SqlConnector` from loopback-connector as the base class for the
MySQL connector.

Use `dataSource.modelBuilder` instead of `juggler.ModelBuilder` to
access the function for registering new schema type.

Note: the behaviour remains backwards compatible, the connector can be
used with both old 1.x and upcoming 2.x versions of
loopback-datasource-juggler.