Split the package into nats-client and nats-client-lwt packages

[dx3mod]

Related #21 issue.

In the ecosystem it is good practice for a library to have support for different I/O libraries like: just unix, lwt, async, eio, etc.

Tasks

• [x] Split the package
• [x] Reuse code

Notes

• Removed specific versions of dependencies.

  Why (in rus). Минимальные версии зависимых библиотек и компилятора мы сможем найти при публикации пакета, на этапе CI оно упадёт и по этим следам мы сможем найти нижнию границу. Описание процесса публикации, но пока рано.

[dx3mod]

Done.

[romanchechyotkin]

@dx3mod i merged testing branch into this to check whether logic is broken, thats why there is new PR #24