client needs to tolerate futures from arbitrary EventLoops

[weissi]

For back-pressure we've got a bunch of callbacks where the user can return futures to make the client continue. Those futures, at the moment, are required to be on the 'right' EventLoop but the client should definitely not require this. Instead, the client should always hop(to:) user futures to the correct EL if it requires it for a certain operation.

In most cases, the user will happen to return futures from the right EL anyway so hop(to:) is pretty much free (it fast paths eventLoop === future.eventLoop and just returns).

[weissi]

example test cases for similar things (initialiser closures on the bootstraps in NIO): https://github.com/apple/swift-nio/blob/master/Tests/NIOTests/BootstrapTest.swift#L94-L209

http client should have similar test cases.