Add `Conduit_mirage.Context`

• Add a Conduit_mirage.Context, a functor for creating HTTP(s) conduit
  contexts (with a DNS resolver).

Fix client-side https resolution for Conduit_mirage

• Fix client-side https:// resolution for Conduit_mirage

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).