[8.x] Fix flushdb for predis cluster

[bernardwiesner]

This is to fix this issue: #20406

Redis::flushdb() or Cache::flush does not work currently with predis cluster, this PR fixes that.

@tillkruss asked me to mention him

[tillkruss]

Thanks @bernardwiesner!

Let's move that logic into the PredisClusterConnection class.

[bernardwiesner]

@tillkruss
thanks I moved it, let me know if its ok now

[taylorotwell]

Please mark as ready for review when all issues have been resolved.

[tillkruss]

LGTM 👍

[tillkruss]

@taylorotwell: The fix here will be going from Client::getConnection() to Client::getIterator() which loops over several Predis\Client instances.

[driesvints]

Please mark this as ready for review when this can be reviewed again.

[tillkruss]

@dries: I removed PredisConnection::flushdb() altogether, which allows new modifiers like:

$redis->flushdb('ASYNC');