Stream entire SSTables when possible

During streaming, Cassandra reifies the sstables into objects. This creates unnecessary garbage and slows down the whole streaming process as some sstables can be transferred as a whole file rather than individual partitions. The objective of the ticket is to detect when a whole sstable can be transferred and skip the object reification. We can also use a zero-copy path to avoid bringing data into user-space on both sending and receiving side.