feat: 🎁 allow Syndicate to group Workers on particular connections

[cratelyn]

a Syndicate is an organized collection of Workers. currently, a single worker always connects to the server, before sending a request and reading the server's response.

often, http/2 servers will handle many requests/responses over a smaller number of long-lived connections, to reduce the overhead of negotiating TLS handshakes.

it would be pleasant for the Syndicate to expose interfaces allowing load-tests to configure the rate of requests and the rate of connections independently.

[cratelyn]

see the relevant section of the http/2 spec, § 9.1.1, here: https://httpwg.org/specs/rfc7540.html#reuse