Make RootTerseUpdatePeriod and ChildTerseUpdatePeriod configurable in [InterestManagement] in OpenSim.ini for experimental purposes.

If n > 1 for RootTerseUpdatePeriod only every n terse update is actually sent to observers on same region, unless velocity is effectively zero (to stop av drift).
If n > 1 for ChildTerseUpdatePeriod only every n terse update is sent to observers in other regions, unless velocity is effectively zero.
Defaults are same as before (all packets are sent).
Tradeoff is reduction of UDP traffic vs fidelity of observed av mvmt.
Increasing n > 1 leads to jerky observed mvmt immediateley for root, though not on child, where experimentally have gone to n = 4 before jerkiness is noticeable.