Add option "aggregate" to customize multiple connections aggregation.

This option must return a callable object that is used to override how
the client aggregates connections when passing an array of parameters
to its constructor.

When specified, this option overrides both "cluster" and "replication"
as it allows to make use of your own code to aggregate multiple nodes.

This is, for example, how you can mimic the standard initialization of
a cluster that relies on client-side sharding:

  $parameters = ['tcp://127.0.0.1:6380', 'tcp://127.0.0.1:6381'];

  $options = [
    'aggregate' => function () {
      return function ($parameters, $options) {
          $connection = new Predis\Connection\PredisCluster();
          $options->connections->aggregate($connection, $parameters);

          return $connection;
      };
    },
  ];

  $client = new Predis\Client($parameters, $options);

When invoked by the client, the specified callable must always return
a Predis\Connection\ConnectionInterface instance or the client will
throw an UnexpectedValueException.
[tests] Rename just a couple of test methods.
[tests] Add @medium since this could take a little bit more than 1sec.
Fix URI parsing on multiple equal signs in pair value of query string.