usrloc: option to set datetime columns as bigint

- should handle better daylight shifting, reported by Alistair
  Cunningham
- new parameter expires_type - if set to 1, it expects expires and last
  modified columns to be bigint
- the change to the type of columns has to be done manually to the
  database server
- default values is 0, expecting to work with datetime columns (existing
  behavior)
usrloc: documented expires_type parameter