2.x: failing IT - testBufferedMutatorPerformsMutations[mutateConcurrently: true] on MirroringConnection client from HBase 2.x

[mwalkiewicz]

[ERROR] testBufferedMutatorPerformsMutations[mutateConcurrently: true](com.google.cloud.bigtable.hbase.mirroring.TestBufferedMutator)  Time elapsed: 22.388 s  <<< ERROR!
java.lang.NullPointerException
        at org.apache.hadoop.hbase.CellUtil.cloneValue(CellUtil.java:126)
        at com.google.cloud.bigtable.hbase.mirroring.TestBufferedMutator.verifyRowContents(TestBufferedMutator.java:190)
        at com.google.cloud.bigtable.hbase.mirroring.TestBufferedMutator.testBufferedMutatorPerformsMutations(TestBufferedMutator.java:169)

Introduced in #108