Correct volatility markings of a few json functions.

 Correct volatility markings of a few json functions.
 
 json_agg and json_object_agg and their associated transition functions
 should have been marked as stable rather than immutable, as they call IO
 functions indirectly. Changing this probably isn't going to make much
 difference, as you can't use an aggregate function in an index
 expression, but we should be correct nevertheless.
 
 json_object, on the other hand, should be marked immutable rather than
 stable, as it does not call IO functions.
 
 As discussed on -hackers, this change is being made without bumping the
 catalog version, as we don't want to do that at this stage of the  cycle,
 and  the changes are very unlikely to affect anyone.