Add mexists method that allow to check the existance of multiple keys

[GustavoCaso]

When calling exists with multiple keys, the return value depending on the arguments could be either a Boolean or an Integer.

This PR aims to fix that by creating a new method called mexists that accepts an array of keys

Example of exists

redis.set("foo", "world")
=> "OK"
[3] pry(main)> redis.exists(['foo', 'foo2'])
=> true
[4] pry(main)> redis.exists(['foo3', 'foo2'])
=> false
[5] pry(main)> redis.exists(['foo', 'foo2'])
=> true
[6] pry(main)> redis.set('foo2', 'world')
=> "OK"
[7] pry(main)> redis.exists(['foo', 'foo2'])
=> 2

Example of exists

redis.set("foo", "world")
=> "OK"
[3] pry(main)> redis.mexists(['foo', 'foo2'])
=> 1
[4] pry(main)> redis.exists(['foo3', 'foo2'])
=> 0
[5] pry(main)> redis.set('foo2', 'world')
=> "OK"
[7] pry(main)> redis.exists(['foo', 'foo2'])
=> 2

[rafaelfranca]

Tests are broken. Can you take a look?

[byroot]

There's also a backward compatibility concern here, I think that's why previous similar PRs stalled:

• Change #exist behaviour for multiple keys #698 #728
• Redis::exists should no longer use Boolify #698
• Adding exists_count #741

[byroot]

@GustavoCaso the CI failure is legit (mexist keys hashing to different servers)

[GustavoCaso]

Looking into it

[byroot]

Fixed in 3257527