Use EncodeDateTime instead of to_char to render JSON timestamps.

Per gripe from Peter Eisentraut and Tom Lane.

The output is slightly different, but still ISO 8601 compliant: to_char
doesn't output the minutes when time zone offset is an integer number of
hours, while EncodeDateTime outputs ":00".

The code is slightly adapted from code in xml.c
Added more regression tests, code comments, documentation, psql tab-complete and check for FK constraint against itself
merge master