Convert from timestamp to datetime. Datetime is consistent between mysql 4.0x and newer

versions and is faster for date/time computations. Modify object creation methods to specify
any time fields that were previously updated with timestamp properties.

tables/session_list.sql

@@ -8,6 +8,6 @@ CREATE TABLE session_list (
  ip varchar(64) default NULL,
  data text,
  messages text,
- stamp timestamp(14) NOT NULL,
+ stamp datetime NOT NULL,
  PRIMARY KEY (session_id)
 ) TYPE=MyISAM;