[Feat] Improve parallel support for v.surf.rst deviation and cross-validation

[petrasovaa]

Is your feature request related to a problem? Please describe.
When running v.surf.rst, multi-threaded computing of deviations and cross-validation is disabled, because it was not implemented.

Describe the solution you'd like
Finish implementation of openmp support. That would include making sure writing to database is not done in parallel.

Describe alternatives you've considered
The functionality works with nprocs=1.

[echoix]

I think with @cyliang368 's work in #3590 this can be considered done?