Cleanly separate IDL annotations from OVSDB schema information.

Cleanly separate IDL annotations from OVSDB schema information.

Until now, the OVSDB IDL annotations have been glommed together with the
schema information in a single file, and then we've used ovsdb-idlc to
extract the schema from that file.  This commit reverses the process:
the schema and the annotations are stored separately and then glommed
together as necessary at build time.

This new arrangement has a few advantages:

    - We can now easily have multiple different sets of IDL annotations
      for a single OVSDB schema.  For example, some users may not need
      access to columns that other users do.

    - Bugs in ovsdb-idlc cannot screw up the underlying schema (as shown
      by a recent commit).