Restoring embedded Cassandra adapter

This reverts commit f0c9468dd3e16779098faebc28f21c3fce834d4c.

It revives the Cassandra embedded storage backend and its tests.  This
is not a pure revert.  To make the code compile, I had to make a few
changes atop the revert to account for refactorings in the interim.
Renaming Groovy-based performance test

Was: GroovySerialTest.groovy
Now: TitanGraphSerialSpeedTest.groovy

I chose the new name because it fit better with the pattern of the
other performance-related graphdb tests, and because I'm about to
propagate this to charting where it will be annoying to modify.
Refactor JaCoCo configuration in toplevel pom.xml

Before this commit, enabling a top-level build profile (such as
memory-test or performance-test) would disable the default-props
profile that I used to set an empty default value for a JaCoCo-related
surefire argLine property.  This commit reworks the surefire config
and attendant maven properties to avoid that behavior.
Fix broken TitanGraphConcurrentTest method

The test testStandardIndexVertexPropertyReads left open transactions
besides the class member named tx.  This commit foregoes opening those
transactions in the first place, instead sharing the class member tx
across several concurrent methods.

This commit also makes the test method wait for the runnables it
submits to an executorservice to complete by get()'ing their futures.
This was previously racy: tearDown() would wait five seconds for the
executor's tasks to complete and then it would throw an exception.
Now the method waits until all the tasks it started are complete
before returning and allowing control transfer to tearDown().
Fix JUB roundcount for MultiQuery perftests