Ocaml Kafka Versions Save

CHANGES:

• Add Async bindings
• Split sync and Lwt variants into separate OPAM packages
• Fix compatibility with OCaml 4.09
• Register callbacks as GC roots
• Port to dune
• Port to OPAM 2 format
• Make dune generate opam files automatically
• Add Changelog to facilitate releases with dune-release
• Depend ob zlib depext instead of explicit package names

Fix build and packaging

Fix the opam file in order to use opam publish.

Fix the package name to match the name actually used by OPAM.

Bug fixes:

• Replace use of int32 by int32_t to be compatible with OCaml 4.03.0.
• Kafka_lwt.consume_batch is not thread-safe
• rd_kafka_conf leak

Internal:

• Port to jbuilder

Support for asynchronous consumers using Lwt.

Support to write Kafka consumers and producers