...to make it easier in the future to replace the binary rdb format with
something else, but also keep support for the old format for backwards
compatibility (extensions).

This should have no performance impact, as the type description manager (a)
caches information about requested type descriptions, and (b) has been changed
to process the bootstrap rdbs en bloc without doing costly consistency checks
(which are useful though when inserting an rdb when installing an extension, but
which would exhaustively read all type descriptions from the inserted rdb, so
would negate any benefit of constructing any type descriptions on demand only).

Change-Id: I80b22770bd9a5e0ab686f04d9c70295f2e3d0bf6