Free the contexts holding this data after we're done using it, by the
expedient of attaching them to the PostmasterContext which we were
already taking care to delete (and where, indeed, this data used to live
before commits e5e2fc84 and 7c45e3a3).  This saves a
probably-usually-negligible amount of space per running backend.  It also
avoids leaving potentially-security-sensitive data lying around in memory
in processes that don't need it.  You'd have to be unusually paranoid to
think that that amounts to a live security bug, so I've not gone so far as
to forcibly zero the memory; but there surely isn't a good reason to keep
this data around.

Arguably this is a memory management bug in the aforementioned commits,
but it doesn't seem important enough to back-patch.