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