Change signature of the connection parameters class constructor.

Only named arrays are explicitly accepted now, but the old behaviour
of creating a connection parameters instance out of an URI string or
a named array is still available using the "create()" static method.

  $array = ['host' => '127.0.0.1', 'timeout' => 1];
  $uri = 'tcp://127.0.0.1?timeout=1';

  $parameters = new ConnectionParameters($array);      // Arrays only
  $parameters = ConnectionParameters::create($array);  // Arrays OK
  $parameters = ConnectionParameters::create($uri);    // Strings OK

The purpose of the change is to have a more concise constructor with a
well defined signature.