Merge pull request #56 from sajinidesilva/slots_Imp

slot Impl - implemented to read missing messages when all the acks are received
package name moved to proper place
Merge pull request #57 from Asitha/patch-1

Package name moved to proper place
Use cassandra-driver-core instead of cql-driver
Merge pull request #58 from a5anka/cassandra_driver_change

Use cassandra-driver-core instead of cql-driver
SlotDeliveryWorker modification - if empty slot is assigned, delete it there