Update `RedisOperations#convertAndSend` to return number of subscribers

[vpavic]

Currently, RedisOperations#convertAndSend is void event though the underlying RedisPubSubCommands#publish returns a Long that represents the number of subscribers receiving the published message (see https://redis.io/commands/publish).

The information about the number of subscribers can be useful in some scenarios, and is also returned by the reactive counterpart in ReactiveRedisOperations#convertAndSend.

Digging through the history of RedisOperations, it seems that #convertAndSend used to return Long for a brief period of time in 2013: it was introduced in b35f6a3 and reverted in d46bee7.

I understand that adding return value would be a breaking change, but the impact doesn't seem to be too big (?). I'm willing to submit a PR to address this, if the change is welcome.

[mp911de]

Thanks for the report. Seems the method signature was reverted to keep binary compatibility. We could consider the change for the next major revision of Spring Data Redis.