Fixes phpredis persistent connections when clients connect to different DBs

[andreausu]

This fixes the corner case where clients connecting to different redis databases are used with persistent connection enabled by giving a unique ID to the persistent connections.

eg:

config:

snc_redis:
    clients:
        db1:
            type: phpredis
            alias: db1
            dsn: redis://localhost/1
            logging: %kernel.debug%
            options:
                connection_persistent: true

        db2:
            type: phpredis
            alias: db2
            dsn: redis://localhost/2
            logging: %kernel.debug%
            options:
                connection_persistent: true

code:

$db1 = $this->getContainer()->get('snc_redis.db1');
$db2 = $this->getContainer()->get('snc_redis.db2');
$db1->set('cachekey1', 'cachevalue');
$db2->set('importantkey2', 'importantvalue');
$db1->flushDb(); // db 2 will be flushed instead of db 1 

From phpredis documentation:

$redis->pconnect('127.0.0.1', 6379, 2.5, 'x'); // x is sent as persistent_id and would be another connection the the three before.

[snc]

Closing this in favor of #308.