Add composable future support (next major release)

[sharonovd]

Java 8 introduced CompletableFuture - more powerful primitive, which allows to chain async operations in a non-blocking fashion using future.andThan.

It would be nice to have such possibility to have this support onboard e. g. via ComposableAsyncOps.

The idea is taken from this fork:
https://github.com/peterservice-rnd/tarantool-java/commits/master

Relevant commits are:
peterservice-rnd@8362063
peterservice-rnd@9b4d03c

I suggest to rebase this commits on top of current master, squash, add tests, relevant build/test targets, and release a major (since the change would be backward-incompatible) version.

[Totktonada]

Assigned Dmitri for the record.

[Totktonada]

Decided to release it as the next minor release (1.9.0), because the change is quite small and should not affect much users.