Add a blocking task executor so that we can easily bound the number of tasks kept live in memory at once

[poodlewars]

Apply it to the finalize_staged_data process as a first example.

There are simpler ways to implement this in do_compact specifically (like completing a future before submitting a new one), and other places in the code use folly::window to do this, but this gives us a good way to handle these problems in a cross-cutting way. We might want to use the blocking queue more widely if this is successful.

This helped to reduce the memory used by finalize_staged_data with write delays, I'm going to do some more benchmarking then share the results here.

Monday: 8234437666