[feature removal] Remove experimental Tarantool engine

[FZambia]

Tarantool engine does not receive updates since it was introduced and now lacks several features - such as idempotent publishing, delta compression. We are aware only about 2 setups where it's used and both clients now moving away from Tarantool engine. Usage stats do not show any notable usage of Tarantool engine also.

It's impossible to properly maintain it given the current Centrifugal Labs resources, so we plan to remove Tarantool engine in Centrifugo v6.