Merge branch 'ps/odb-generalize-prepare'

Merge branch 'ps/odb-generalize-prepare'

The 'reprepare()' callback for object database sources has been
generalized into a 'prepare()' callback with an optional flush cache
flag, and a new 'odb_prepare()' wrapper has been introduced to allow
pre-opening object database sources.

* ps/odb-generalize-prepare:
  odb: introduce `odb_prepare()`
  odb/source: generalize `reprepare()` callback