odb: only freshen pack files every 2 seconds
Since writing multiple objects may all already exist in a single
packfile, avoid freshening that packfile repeatedly in a tight loop.
Instead, only freshen pack files every 2 seconds.
odb: freshen existing objects when writing
When writing an object, we calculate its OID and see if it exists in the
object database. If it does, we need to freshen the file that contains
it.