Add ORC checkpoints

Only flush buffered values if necessary
Fix Hive partition pruning for null keys

Fixes #1864
Remove unused StorageManager H2 database
Do not write CRC files for Raptor
Cleanup Raptor staging directory
Update cassandra driver to 2.1.0

The 2.0.0-rc2 driver has a bug
(https://datastax-oss.atlassian.net/browse/JAVA-254) which gets
triggered when querying through presto.  An exception occurs because the
driver attempts to query a table that no longer exists, and fails.

Cassandra 2.1.0 is the current stable release, so it's probably time to
catch up.  The hector library has been removed and replaced, because it
didn't seem to work correctly with C* 2.1, and the project doesn't
appear to be very active.
Fix formatting