Add support for AT LOCAL

The Standard defines time zone conversion as follows:

<datetime factor> ::=
<datetime primary> [ <time zone> ]

<time zone> ::=
AT <time zone specifier>

<time zone specifier> ::=
LOCAL
| TIME ZONE <interval primary>

While looking at something else, I noticed we do not support AT LOCAL.
The local time zone is defined as that of *the session*, not the server,
which can make this quite interesting in views where the view will
automatically adjust to the session's time zone.

Patch against 3f1aaaa180 attached.
--
Vik Fearing