Refactor continuous aggregate materialization code

In addition to the code reorganization and simplification we changed
it by splitting the query execution in multiple steps taking advantage
of `SPI_prepare`, `SPI_execute_plan` and `SPI_freeplan`:
* create_materialization_plan(PlanType)
* execute_materialization_plan(PlanType)
* free_materialization_plan(PlanType)

This PR is in preparation for a following PR to execute the
materialization in small batches to alleviate the I/O spikes when
reading and writing many buckets.