[Database] Allow configuration of polling interval

Reduces database polling interval, and allows
configuration on a per-user basis. This addresses
both perceived and actual issues with object
modification:

* nasa/mct#54 notes that object deletion seems
  to take a long time. Root cause is that the
  Browse area is refreshed by database polling,
  which takes place at a longer interval (3s).
  Decreasing this interval means that the user
  sees the consequences of their action sooner.
* nasa/mct#132 occurs when the user switches
  back and forth between objects faster than
  the poll interval. This can cause an object
  to be flagged as stale because changes from
  one save have not made their way back to
  the Browse area. While this still may occur
  with a shorter polling interval, the odds
  of the user observing this decrease
  significantly.
* nasa/mct#111 describes latency issues, some
  of which may be related to the poll
  interval. (It is unlikely, however, that
  menu item latency is related to this.)