Expose async tls_handler

[mbacarella]

I proprose this PR to expose the async tls_handler to users of the library. This is needed if you want to do your own Tcp service management, such as if you're plugging async tls support into other frameworks.

[hannesm]

that's a pretty huge diff for the description. any chance you could remove any code formatting changes from this PR?

also, maybe @torinnd could take a look at the changes?

[torinnd]

I'd probably recommend collapsing the new test_server_low_level example into the test_server. They don't look different enough to me to justify persisting both.

I also think I'd refactor the MLI as follows:

type 'a io_handler = Reader.t -> Writer.t -> 'a Deferred.t
type 'a tls_handler = Session.t -> 'a io_handler

val upgrade_server_handler
  : config:Tls.Config.server
  -> 'a tls_handler
  -> 'a io_handler

Notably this excludes any mention of the socket address from the type signature as that isn't relevant to the TLS upgrade being performed here. Callers would probably do something like upgrade_server_handler ~config (my_cool_handler socket) to satisfy the Async.Tcp.Server.create interface.

[mbacarella]

Thanks for the review. Collapsing the test servers into one and the .mli refactor done.

I believe all of the extraneous code formatting has been removed (mostly in the dune file).

[hannesm]

Thanks for your contribution. I squashed and merged, this will be part of the next release.