Add Dataset#current_datetime for returning current Time/DateTime object

The 3 plugins that needed a current datetime value were inconsistent,
with one caling Sequel.datetime_class.now and the other two calling
Time.now.  This makes sure that Sequel.datetime_class is used in
both places.  It also makes it possible to override the method to
change the behavior for all three of these plugins.
Add Dataset#literal_{datetime,time,sqltime}_append

This changes literal_append to call these new append methods.
No user visible changes except for a slight performance
decrease due to extra method calls, which probably will not
be noticeable considering the speed of strftime.
Add current_datetime_timestamp extension, for current Time/DateTime instances that are literalized as CURRENT_TIMESTAMP

This allows you to have correct transactional behavior for current
timestamps, while still using a Time/DateTime API-compatible
object at the ruby level (allowing things like typecasting to work
correctly).