Provide a type which offers concurrent keyed and queued execution of tasks.

Something that looks like:

KeyedExecutor<std::string> ke(someExecutorMaybeThreadPool);
auto doThingFuture = ke.execute("foo", []{ return doThing(); });
auto doOtherThingFuture = ke.execute("foo", []{ return doOtherthing(); });
auto waitForCurrentFooState = ke.waitForAll("foo");

Where:

• Jobs under the same key provide mutual exclusion
• Tasks are executed via the passed executor
• running jobs returns futures
• waiting for a queue to clear up to a point is based on the back of the queue at the time of the call's future.