CPS: Force establish a connection with a short timeout for _resolve/index + other tiny refactorings

[pawankartik-elastic]

This PR contains the following changes:

1. Swaps out an inappropriate disconnected strategy with a connection strategy that's ideal for CPS (and previously used in TransportSearchAction),
   • We'd want to wait for a short duration and bail out if a linked project is unresponsive. This is a prerequisite as we plan on adding tests that exercise this scenario in the near future.
2. Swaps out unnecessary usage of SortedMap,
3. Swaps out the combination for CountDown and Runnable in favour of GroupedActionListener, which essentially does something similar under the hood (this cuts down unnecessary exposed boilerplate code),
4. Removes the temporary assertion on the number of requests fanned out and the responses received — a more appropriate fix is approved and pending merge.
   • Edit: this change was merged in: Do not assume we hear back from all linked projects when validating resolved index expressions for CPS #137916.

[pawankartik-elastic]

I can move 3L out in favour of a final class member.