Avoid is_coroutine_function calls in Server

We currently support handlers that are either synchronous or asynchronous

https://github.com/dask/distributed/blob/1297b18ff09276f6ad1553d40ab3ce77acf0fc0e/distributed/core.py#L567-L572

In #4443 we learned that these checks take up some time. In a recent run it takes 100ms out of about a 2.5s run, so around 4%

There are a couple of ways to address this:

1. We can try to find some other cheaper check
2. We can make all of our handlers `async def` functions and asyncify the entire codebase (we should also check that this doesn't incur other costs if so)

cc @dask/scheduler-performance