Fixes#604
Ensures that extra seeds are propagated to all crawler instances.
Adds a new redis hashmap key to store the extraSeed mappings
url->extraSeeds index, to ensure the extra seeds are added in the same
order on other instances, even if encountered in different order.
Add a new redis lua primitive 'addnewseed' which combines several
operations: check if extra seed already exists and returning existing
index, add new seed to extraSeed list, also add to regular URL seed
list.
