The thread_locals are not great (they're slow in dynamic builds).
There's a deque that's constructed but never used except on worker threads: https://github.com/10gen/mongo/blob/dab0694cd327eb0f7e540de5dee97c69f84ea45d/src/mongo/transport/service_executor_synchronous.cpp#L103

There's another one that's not used at all.

The data members could be more cleanly structured to make this complex class more followable.