Reduce file descriptor usage

[jkilpatr]

An instance of rita uses more than ten thousand file descriptors in it's normal state. That's crazy and needs to be reduced.

Just looking as lsof it seems to be mostly async/await related.

[jkilpatr]

Some experimentation here shows that this is probably the number of independent async/await executors we have running in various threads. Separating out the executors has been useful for preventing unexpected blocking behavior from causing issues in unrelated parts of the program by hotting the executor thread but clearly is coming at a cost.