The command execution path for Mongos starts from here, and ends in the invocation of CommandHelpers::runCommandInvocation (here and here).

To add support for asynchronous command execution, this path should replace inline executions with returning futures that can be scheduled on an executor.

Next, invocations of CommandHelpers::runCommandInvocation should be replaced with CommandHelpers::runCommandInvocationAsync to allow utilizing async implementations of commands, when available.