deps: update futures to 0.3.15 (#1022)

This branch updates the `futures` crate to v0.3.15. This includes a fix
for task starvation with `FuturesUnordered` (added in 0.3.13). This may
or may not be related to issues that have been reported in the proxy
involving the load balancer (linkerd/linkerd2#6086), but we should
update to the fixed version regardless. This may also improve
performance in some cases, since we may now have to do fewer poll-wakeup
cycles when a load balancer has a large number of pending endpoints.

Cargo.toml

@@ -10,7 +10,7 @@ publish = false
 retain = ["linkerd-stack", "tower"]
 
 [dependencies]
-futures = "0.3.9"
+futures = "0.3.15"
 tokio = { version = "1", features = ["macros", "sync"] }
 
 linkerd-stack = { path = "../stack", optional = true }