Don't send empty dependencies

[Kobzol]

When a client sends the update-graph message, it sends a dictionary with dependencies of individual tasks. Tasks without dependencies needlessly occupy bandwidth in this message, since the scheduler anyway does something like dependencies.get(task, ()) when it parses the message (if I'm wrong please let me know).

For example for this simple merge with a lot of tasks without dependencies:

from dask import delayed
from dask.distributed import Client

client = Client("tcp://localhost:8786")

@delayed
def do_something(x):
    return x * 10

@delayed
def merge(*args):
    return sum(args)

xs = [do_something(x) for x in range(1000)]
result = merge(*xs).compute()

This PR changes the dependencies in the update-graph message from this:

{
    "merge-3cfcfa4f-fa40-45ba-b181-54eb7554377f": ('do_something-ae852807-355c-4d87-85e4-72e6f1d0a146', 'do_something-3e003b98-363b-42ab-b270-691db098f222', ...),
    "do_something-9f9820f5-fbe9-4d31-ba9d-ac17996e4c35": (),
    "do_something-34c2c63b-2d3d-4e5c-8fef-f94ab635e19f": (),
    ... # 998 additional lines
}

into this:

{
    "merge-3cfcfa4f-fa40-45ba-b181-54eb7554377f": ('do_something-ae852807-355c-4d87-85e4-72e6f1d0a146', 'do_something-3e003b98-363b-42ab-b270-691db098f222', ...),
}

For the aforementioned graph, it reduces the update-graph total frame byte size by the following amounts:

Merge task #	Original size	Reduced size	% of original
5000	4170630 B	3910628 B	94 %
10000	8340666 B	7820664 B	94 %
15000	12510702 B	11730700 B	94 %
20000	16680738 B	15640736 B	94 %
25000	20850774 B	19550772 B	94 %

[mrocklin]

Thank you for the pull request @Kobzol . I'm curious, was this work motivated by something in particular? The performance changes here are small, so I'm trying to decide if adding this change is worth the added complexity. Was there a motivating application?

[Kobzol]

While working on the Rust scheduler, I noticed that there is a lot of empty dependencies being sent, which seemed inefficient to me. I wouldn't send this PR if it was a complicated change, but since it's a one liner, I thought that it's a nice improvement for such a small change.

I later had to modify the Graphlayout to work with tasks instead of dependencies, but even that doesn't seem like a big change.

[mrocklin]

Thanks @Kobzol ! Sorry for the delay