Enable the use of SyncedVariantReader with no intervals

Specify "all intervals" by passing in an empty string.
The & is missing because it's using copy-by-value-and-move :)

There's a default interval argument in the iterator class.  I can move it to here.  But note that this is <b>Indexed</b>VariantReader so I really shouldn't have touched it in this commit anyway.  I'll pull it out of here and submit a different one.