Fix mismatch of blocking calls inside asynchronous methods

[danielballan]

This method is asynchronous but performs a blocking write!

https://github.com/bluesky/tiled/blob/main/tiled/adapters/zarr.py#L148-L156

The read* and write* methods are allowed to be synchronous or asynchronous; when called by the server they are wrapped in a helper function that pushes synchronous methods to a thread:

tiled/tiled/utils.py

Lines 736 to 742 in 0388f21

	async def ensure_awaitable(func, *args, **kwargs):
	if is_coroutine_callable(func):
	return await func(*args, **kwargs)
	else:
	# run_sync() does not apply **kwargs to func
	# https://github.com/agronholm/anyio/issues/414
	return await anyio.to_thread.run_sync(functools.partial(func, **kwargs), *args)

But of course this mechanism is thwarted when we put synchronous I/O code inside an asynchronous method. This kind of bug seriously harms the scalability of the server, as the entire event loop will grind to a halt while processing this request.

The methods on this Adapter and others should be reviewed for correctness.

[danielballan]

A quick git grep shows that the problem is, fortunately, isolated to just this one adapter:

❯ git grep "async def" tiled/adapters/
tiled/adapters/zarr.py:    async def write_block(
tiled/adapters/zarr.py:    async def patch(