ST_AsMVT parallel implementation

[pramsey]

Using the serialize/deserialize infrastructure already in the protobuf format, we can relatively easily pass a partial aggregate from a worker node to a master node for combination into the final output. The only potential downside is that the MVT process itself is not much of a bottleneck, it's more the process of building the transformed geometries that takes time, so potentially this is trying to force parallelism on other aspects of an MVT query.

[pramsey]

In 16105:

Parallel implementation of ST_AsMVT (Closes #3927)