db_handle reform
Use "ns_db currenthandles" (when available) to implement "db_with_handle"
instead of the old approach based on the global db_state variables.
The enws approach has the advantantge that it is
- more robust (deletion and creation of the per-request variables,
no coherency problem),
- simpler, and
- faster (less overhead per db_with_handle call, simple queries up to 20% faster)
time {db_string . {select object_id from acs_objects limit 1}} 1000
old: 200-230 microseconds per iteration
new: 160-180 microseconds per iteration
