v3: replace channel in Future with a cond.Sync

[oleg-jukovec]

The channel:

go-tarantool/future.go

Line 19 in e72726b

done chan struct{}

Is used to notify a waiter about a response. It could be easily replaced with a condition variable + boolean value.

But it's not that simple. We also have a method that returns a channel to the user. In this case we need to create the channel on demand:

go-tarantool/future.go

Lines 127 to 133 in e72726b

	// WaitChan returns channel which becomes closed when response arrived or error occurred.
	func (fut *Future) WaitChan() <-chan struct{} {
	if fut.done == nil {
	return closedChan
	}
	return fut.done
	}