`dask.distributed.Lock` always async

**What happened**:
Using an asynchronous `dask.distributed.Client` both the asynchronous and synchronous context managers are asynchronous. Producing a `coroutine 'PooledRPCCall.__getattr__..send_recv_from_rpc' was never awaited` at runtime.

**What you expected to happen**:
I would intuitively always expect async functions to be asynchronous and regular functions synchronous.

**Minimal Complete Verifiable Example**:

```python
import asyncio

from dask.distributed import Client
from dask.distributed import Lock

async def lets_go(client):
print("requesting a")
async with Lock(name="a", client=client):
print("acquired a")
print("released a")

print("requesting b")
with Lock(name="b", client=client):
print("acquired b")
print("released b")

async def main():
async with Client(asynchronous=True) as client:
await lets_go(client)

if __name__ == "__main__":
asyncio.run(main())# Put your MCVE code here
```

**Anything else we need to know?**:

**Environment**:

- Dask version: 2022.5.0
- Python version: 3.9
- Operating System: macOS
- Install method (conda, pip, source): pipenv