Port to jbuilder and remove depopts

Rearrange the opam packages to eliminate optional dependencies,
and have explicit and separate packages for the Lwt, Async and
Mirage implementations. The opam and ocamlfind layouts now
have the same names:

• dns: the core package
• dns-lwt: Lwt implementation
• dns-lwt-unix: Lwt Unix, including servers
• dns-async: Async implementation (this currently uses Unix)
• mirage-dns

This layout is not compatible with the older releases which had
ocamlfind subpackages, so an upper bound will be needed in OPAM
for those. However, porting should be relatively straightforward
to the new release, and in return your users will not have to deal
with specifying a myriad of optional dependencies in OPAM.

• This release also ports the build to use Jbuilder, which speeds
  it up quite a bit and removes boilerplate files.

• Depend on Lwt 3.0.0 interfaces, including the blocking bind.