Track field changes to allow cache optimisations

[rchristie]

The FindMeshLocation field relies on a cache of host coordinate ranges which currently isn't built and used if that field is marked as changed.
If Zinc could track the state e.g. a change counter, at which the field (or any fields it depends on) were last changed relative to when the the coordinates range was built, the optimisation could be used even during begin/end change with active modifications on the field.