clippy.toml: Create and disallow unbounded channels

When using channels (e.g. `futures::channel::mpsc` or `std::sync::mpsc`)
always use the bounded variant, never use the unbounded variant. When
using a bounded channel, a slow consumer eventually slows down a fast
producer once the channel bound is reached, ideally granting the slow
consumer more system resources e.g. CPU time, keeping queues small and
thus latencies low.  When using an unbounded channel a fast producer
continues being a fast producer, growing the channel buffer
indefinitely, increasing latency until the illusion of unboundedness
breaks and the system runs out of memory.

One may use an unbounded channel if one enforces backpressure through an
out-of-band mechanism, e.g. the consumer granting the producer
send-tokens through a side-channel.

See also
libp2p#2780 (comment)

clippy.toml

@@ -0,0 +1,3 @@
+disallowed-methods = [
+  { path = "futures::sync::mpsc::channel", reason = "does not enforce backpressure" },
+]