make Server::start infallible and add fn builder()

[niklasad1]

Server::start was infallible to let's change the API to reflect that.

[jsdw]

LGTM! The only reason to keep the result would be if we thought it might become fallible again, but best not to anticipate such things anyway :)

[niklasad1]

Yeah, I'm quite tempted of removing the entire start API then just start the server from the Builder but it may useful to manage when the server is started, not sure.

[jsdw]

Having a guide gander, it looks like you may as well keep a separate thing to start it offhand; I tend to feel that breaking functionality apart like this is a good thing (and then providing higher level stuff to do the common cases where applicable)

(I'd probably add a Server::builder() method though as a random thought because that might be a little cleaner than importing a separate Builder type to use to build it, and if I'm looking to make a thing I usually don't think to look for a separate type to construct it)

[niklasad1]

I will remove the start API in another PR.

It's quite messy to migrate all tests.