Persistent mode fix

[faseelmo]

Refer: Issue #339

Fixes:

1. The Persistent mode value was incorrectly documented as -1 in the README. Updated the documentation to the correct value which is 2.
2. GetTemporalClearingDuration() previously returned the voxel_decay parameter value. In Persistent mode, when voxel_decay is unset, this function returned -1 (default), causing voxels to be cleared immediately. This resulted in no objects being displayed in the costmap. The function now returns std::numeric_limits::max() in Persistent mode.