rename equiv package to equivalence
add better content ref types
make Worker generic
Add EquivalenceGraph, Store and Abstract implementation
Introduce ContentRefSerializer

to replace ChildRefSerializer and SeriesRefSerializer
Add CassandraEquivalenceGraphStore
LegacyMessageSerializer can handle ContentEquivalenceAssertionMessage
datastaxservice can take clusterbuilder
Create EquivalenceGraphStore in CassandraPersistenceModule
create worker to update equivalences
fix zombie equivalence graph rows
Switch to protobufs for CassandraEquivalenceGraphStore