Make ResultSet serializable so cached queries can be executed

[andrii-pukhalevych]

Calling Query::cache() and then executing the query always threw "Serialization of 'CurlHandle' is not allowed", which made query result caching unusable with FileEngine.

Query::all() hands the decorated ResultSet to QueryCacher::store(), and ResultSet keeps the query that produced it in $queryObject. From there the whole connection is reachable:

  ResultSet::$queryObject      -> Query
  Query::$_repository          -> Index
  Index::$_connection          -> Datasource\Connection
  Connection::$_client         -> Elastica\Client
  Elastica\Client::$_transport -> Transport
  Transport::$client           -> Transport\Client\Curl
  Curl::$curl                  -> CurlHandle

Add __serialize()/__unserialize() that leave the query object out.
The decorated Elastica\ResultSet is kept whole rather than flattened to documents, so getTotalHits(), getAggregations(), getSuggests(), getMaxScore(), getTotalTime() and hasTimedOut() still answer correctly after a cache round trip. Since unserialize() does not run the constructor, __unserialize() calls parent::__construct() to set up the decorated iterator.