Support ocaml-tls in mirage, rework the mirage API

• Full support for ocaml-tls.0.5.0
• Breaking API change for mirage-conduit. Now all the flows are dynamic,
  the functors are becoming first-class values so no big functor to build
  first.

Set TCP_NODELAY on unix domain sockets

• Partial support for ocaml-tls.0.5.0
• setsockopt TCP_NODELAY fails on a Unix domain socket (#63 by @djs55)

Make Mirage_TLS an optional dependency

Make TLS optional in Conduit_mirage, and disable it by default so that it is a developer-only option until it is properly released. It can be enabled by setting the HAVE_MIRAGE_LWT env variable.

Plug Unix resource leaks and support latest Async_ssl

• Support Async_SSL version 112.24.00 and higher.
• Add a TLS echo server in tests/async/
• [lwt] Do not leak socket fd when a connect or handshake
  operation fails (#56 via Edwin Torok).
• [async] Do not leak pipes in SSL handling (#54 from Trevor Smith).

TLS compatibility

• Add TLS client support for Mirage (#50)
• Do not overwrite the default name resolver for Mirage (#49)
• Add TLS support using the pure OCaml TLS stack (#46).
• Replace the Mirage Make_flow functor with Dynamic_flow that is
  easier to extend with more flow types.

Add error message display function

• Add an error_message function to simplify error display (#38).
• Improvements to documentation (#37).

Reduce debug logging by default

• Do not emit debug output when the CONDUIT_DEBUG variable is not set.
• Do not create symlinks in a local build, which helps with OPAM pins.
• Improve ocamldoc for Conduit_lwt_unix.

Add native OCaml-TLS support

• Add Lwt-unix support for the native OCaml/TLS stack as an alternative
  to OpenSSL. This can be activated by setting the CONDUIT_TLS environment
  variable to native. If this is not set and OpenSSL is available, then
  OpenSSL is used by in preference to the pure OCaml implementation.
• Add sexp convertors for Conduit_lwt_unix.ctx and Conduit_mirage.ctx
  and the Resolver service types.
• Fix the Mirage tests to the Mirage 2.0.1+ Conduit interfaces.
• Add more debugging output when the CONDUIT_DEBUG variable is set on Unix.
• Interface breaking: The client and server types in Conduit_lwt_unix
  now explicitly label the fields of the tuples with a polymorphic variant.
  This allows them to remain independent of this library but still be
  more self-descriptive (i.e. Port of int instead of just int).

Improve connection closing semantics

When terminating conduits, always close the output channel first before the input channel, so that any pending data in the underlying fd is flushed.

Significant interface improvements for Lwt, Async and Mirage

• Add an explicit ctx content to track every conduit's runtime state.
• Allow the source interface for a conduit to be set.
• Support a password callback for the SSL layer (#4).
• [lwt] Add stop parameters in main-loop of the server (#5).
• Add Conduit_mirage with Mirage functor suport.
• Add ocamldoc of most interfaces.
• Add a CONDUIT_DEBUG environment variable to the Unix backends for
  live debugging.
• Add a conn value to the callback to query more information about the
  current connection (#2).
• Expose the representation of Conduit_lwt_unix.flow in the external signature.
  This lets library users obtain the original Lwt_unix.file_descr when using
  Conduit libraries like Cohttp.