* we must now ignore that datafiles are not sealed
this is because an unsealed datafile may have been produced by
renaming multiple journals to datafiles at server start
* acquire collection count after we have acquired the lock
* count the null byte as well
* fix count value acquisition
* send query fragments to the correct servers, even after failover or when a follower drops
the problem with using the previous shard-based approach is that responsibilities for shards may change at runtime
however, an AQL query must send all requests for the query to the initially used servers.
if there is a failover while the query is executing, we must still send all following requests to the same servers, and not the newly responsible servers
otherwise we potentially would try to get data from a query from server B while the query was only
instanciated on server A.