Some improvements to the hydroflow template

[rohitkulshreshtha]

1. Default server address, so you don't have to copy an address around when starting multiple clients to test a full star network configuration.
2. Help messages for all the arguments.
3. More hand-holding documentation, mostly condensed from the first networking chapter
4. Updated README.md to provide more runnable commands.

[rohitkulshreshtha]

My guess is that cargo fmt --check was added recently? For example, it is failing on

Echo { payload: String, ts: DateTime<Utc> }

which has always existed...

[MingweiSamuel]

Oh and for cargo fmt, just run cargo fmt (or cargo +nightly fmt) and it will fix all the formatting and pass CI

[jhellerstein]

LGTM. Maybe change the earlier advice to use for_each(...println!...) to say inspect(...println!...)?