Some problems with hybrid workload and result collection

[gregn123]

I encountered 2 issues with this benchmark, when running the hybrid workload:

1. Sometimes a NullPointerException occurs on line 683 of main() (HTAPBench.java), because runHybridWorkload() erroneously returns when EITHER tpcc or tpch results are non-null, instead of when BOTH tpcc and tpch are non-null.

2. Sometimes the ClientBalancer considers the system to be saturated before any OLAP streams have been launched, so then there will obviously be no OLAP results.

I created the following pull request with fixes for these issues (to be reviewed).
Fix issues with hybrid workload and result collection