Centralize json and jsonb handling of datetime types

Centralize json and jsonb handling of datetime types

The creates a single function JsonEncodeDateTime which will format these
data types in an efficient and consistent manner. This will be all the
more important when we come to jsonpath so we don't have to implement yet
more code doing the same thing in two more places.

This also extends the code to handle time and timetz types which were
not previously handled specially. This requires exposing the time2tm and
timetz2tm functions.

Patch from Nikita Glukhov