Added basic per-thread time zone functionality (based on public
domain elsie-code). Now user can select current time zone
(from the list of time zones described in system tables).
All NOW-like functions honor this time zone, values of TIMESTAMP
type are interpreted as values in this time zone, so now
our TIMESTAMP type behaves similar to Oracle's TIMESTAMP WITH
LOCAL TIME ZONE (or proper PostgresSQL type).
  
WL#1266 "CONVERT_TZ() - basic time with time zone conversion 
function".
  
Fixed problems described in Bug #2336 (Different number of warnings 
when inserting bad datetime as string or as number). This required
reworking of datetime realted warning hadling (they now generated 
at Field object level not in conversion functions).
  
Optimization: Now Field class descendants use table->in_use member
instead of current_thd macro.