Forward compatibility with upcoming Promise v3

[clue]

Builds on top of reactphp/promise#75, reactphp/promise#213, reactphp/promise#138, reactphp/socket#214 and reactphp/promise-stream#20
Also builds on top of dev updates in clue/reactphp-block#61, clue/reactphp-http-proxy#44, clue/reactphp-socks#106 and clue/reactphp-ssh-proxy#35

[clue]

Rebased to fix merge conflict now that #464 has been merged. There are still a couple of failing tests that look like some unexpected behavior changes in Promise v3 as also noticed by @WyriHaximus in reactphp/socket#214 (comment). Considering the HTTP component depends on the Socket component, let's continue this discussion in the upstream PR. I'll make sure to update this PR once we make progress.

[clue]

Updated to use Socket v1.12.0 and apply the same changes to fix a race in the tests as discussed in reactphp/socket#214 (comment), so the build should now be green. This is still WIP as this currently still depends on updated dev dependencies that I'll try to get shipped no later than next week.

[clue]

Updated to stable releases now that all dependencies have been released:

• https://github.com/reactphp/socket/releases/tag/v1.12.0,
• https://github.com/clue/reactphp-http-proxy/releases/tag/v1.8.0,
• https://github.com/clue/reactphp-ssh-proxy/releases/tag/v1.4.0,
• https://github.com/clue/reactphp-socks/releases/tag/v1.4.0

Also worked around another nasty HHVM build error because legacy HHVM does not support the iterable type in Promise v3 (reactphp/promise#225).

This is now ready for review