Difference between pool and allocator passed to QueryCtx constructor

[devavret]

What is the difference in usage of allocator and pool passed to QueryCtx constructor?
Is the memory allocated by allocator used for temporary usage of operators while pool is used for allocating returnable buffers? In a previous commit, the allocator passed to QueryCtx was used in Task::addOperatorMemory which is why this appeared to be the case. In the most recent commit, the allocator seems to be only used for constructing ConnectorQueryCtx.

Is it on the roadmap to completely remove allocator argument in the QueryCtx? If so, is there another plan to allow passing separate pools for temporary and returnable allocations?