Pull Queries: pre-compile known steps

[big-andy-coates]

We know there are steps needed when working with pull queries, e.g. we need to compile code to handle the final map step of aggregated fields, any HAVING clauses and SELECTs for mapping from the internal to table schemas. We can precompile these when building the aggregate table, rather than on ever pull query request.

(Also, we may be able to avoid the 'final selects' step entirely if we can map the users selects to the internal topic schema...)