pd: 👯 use tokio's JoinSet for polling servers

[cratelyn]

in pd's startup code, we use tokio::select! to poll a bundle of futures, none of which should return until an error is encountered.

this would be a great opportunity to use tokio::task::JoinSet, which has recently been added to the library beneath the rt feature flag.

• https://docs.rs/tokio/latest/tokio/task/struct.JoinSet.html

quoth the docs:

A JoinSet can be used to await the completion of some or all of the tasks in the set. The set is not ordered, and the tasks will be returned in the order they complete.

All of the tasks must have the same return type T.

When the JoinSet is dropped, all tasks in the JoinSet are immediately aborted.

these properties (particularly grouping up tasks to abort upon failure/shutdown) are all what we want, and it exposes (imo) a friendlier interface than the alternate macro syntax.

additionally, it would make it much easier to propagate errors from other subsystems, e.g. ACME events (see #3652), to be fatal.

cc: @conorsch 🔖