Remove pg_auto_parameterize and pg_statement_cache extensions

These extensions were developed for the postgres adapter to see
if it made sense to see the effect of auto parameterization on
Sequel.  My testing showed that in most cases, auto
parameterization led to slow downs, even when caching the
statements.  For the cases where auto parameterization was
faster than not parameterizing, manual paramerization was faster
still, so auto parameterization for performance doesn't make
sense.

I'm not aware of anyone using these extensions in production,
or even being used by someone besides me.
Remove Model#set_values, #save without #_refresh

Model#set_values was originally made public because Model.call
used it to set the values after calling Model.allocate.

Model#set_values was used internally in the following places:

1) Loading records from the database
2) Refreshing a new record after save
3) Manual refreshes via Model#refresh

These will be changed in the following ways:

1) Loading records from the database will use
   instance_variable_set to load the hash into the instance.
   Extensions that want to affect the data stored for model
   objects loaded from the database will have to override
   Model.call.

2) Model#_create_set_values will be added to set values
   for newly created objects.

3) Model#_refresh_set_values will be added to set values for
   manually refreshed objects.

While 2) and 3) will result in some redundant code, the cases
should be treated differently.

Related to this is #save not calling #_refresh for new objects.
It currently doesn't do that now if insert_select is supported,
this will extend that to when insert_select is not supported.
The reasoning behind this is a manual refresh is done to
discard changes made to the object, while a refresh after save
is mostly done to get new values not set in the INSERT, where
existing values should not change.

For example, currently the associations hash is cleared after
save, but in most cases it is cleared unnecessarily, since the
values of foreign keys set before the record is saved have not
changed.

This may require changes to plugins that override #_refresh,
and will definely require changes to plugins that override
set_values.

These changes should improve performance and result in better
behavior.