Set#flush allows to retrieve all records from the set and clear it
(basically, we just pop from it until it is empty)

Set#mass_add adds records from the given array to the set in transaction

Now it support optional parameter (limit), you can specify how many
items you want to pop

We lost ```limit+1``` value from a set when retrieved ```limit``` values
from the set. Fixed that

## Summary
- Fix `Redis#exists(key)` warning 
- Parent repo for reference: https://github.com/nateware/redis-objects/blob/master/lib/redis/helpers/core_commands.rb
- It's a public repo (!)

```
`Redis#exists(key)` will return an Integer in redis-rb 4.3. `exists?` returns a boolean, you should use it instead. To opt-in to the new behavior now you can set Redis.exists_returns_integer =  true. To disable this message and keep the current (boolean) behaviour of 'exists' you can set `Redis.exists_returns_integer = false`, but this option will be removed in 5.0. 
```