Performance test refactoring

This commit adds a new test, TitanWritePerformanceTest, which tests
vertex, edge, and property insertion on an empty graph with and
without batch loading enabled.  The code is descended from
TitanGraphPerformanceTest last seen in
7d69240e324cb5c2bef71a25dc88fbead1697308, but it's been refactored to
use BatchGraph and helper methods in TitanGraphTestCommon for schema
creation, and to de-noisify the edge insertion pattern.

This commit removes several performance test classes:

* the two-method "PerformanceTest" and the TitanBenchmarkSuite class
  that used it.  There were no external references to these classes;
  the only reference appeared to be a block of commented code in
  TitanBlueprintsTest, which was also removed.

* SpeedComparisonBenchmark, which seems mostly subsumed by other test
  classes (except possibly the VertexList retrieval)

* KeyColumnValueStorePerformance -- this is a good idea, but it
  appears to have been barely started and never really finished.  The
  only subclass in the source tree is for the InMemory backend.
  Deleting it for now since the performance data it produces are not
  useful, but maybe we will restore and flesh it out later.

SerializerTest contained a combination of correctness-focused and
performance-focused tests.  I extracted common logic into
SerializerTestCommon, then moved the performance tests to
SerializerSpeedTest (which also bears the PerformanceTests JUnit
category).  The correctness-focused tests are still in SerializerTest.

This commit adds a new enabled-by-default JUnitBenchmarks reporter
that writes a CSV file with one line per test method.  The reporter is
internally synchronized but it isn't safe for use by concurrent
processes (e.g. multiple surefire test forks).  The file path is
target/jub.csv.  The output includes an initial header row.