Activate mem/perf test profiles using properties

Profile activation with -P... has proven brittle and limited because
of the way it interacts with <activeByDefault> (disables all marked
profiles in the same pom but not parent or child poms).  I'm changing
the memory-test and profile-test profile activation criteria.  Now
they activate when -Dtest.mem=true or -Dtest.perf=true are present on
the command line (respectively).  Using -Pperformance-test or
-Pmemory-test is not recommended.
Cleanup titan-cassandra/pom.xml

This commit removes unreferenced properties and reduces the overall
size of the pom by moving the BOP/RP Cassandra test configurations out
of profiles and into the <build> tag of the pom.  Both the BP and RP
tests are enabled by default.  To skip one, set
-Dtest.skip.ordered=true or -Dtest.skip.unordered=true (respectively).
I would rather these properties be named test.ordered and
test.unordered and their boolean sense be negated, but I don't see an
easy way to do it with Maven right now.
Enable assertions in test JVMs forked by Maven
Tweak memory/performance test profile activation
Allow no tests in default surefire execution
Write short doc of available Maven test options