Remove duplicate dependency in presto-hive
Fix race condition when updating output buffers
Fix race condition when updating sources

The current implementation has a potential race condition under the following scenario:

Task updater              Driver

                          acquire lock
                          process new sources
                          ...
                          process new sources in lock release code
add new source
fail to acquire lock
 (sources not processed)
                          release lock

This results in new sources not being processed and the query getting stuck
if the task updater never call update sources again and the driver becomes
blocked because data from all current exchanges are consumed
[cassandra] Bump to current stable version.

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.