[core] Add the TimeSeriesWorkload class, a new type of workload that uses (#1008)
the existing YCSB API to generate time series data (numeric values associated
with timestamps).
See the TimeSeriesWorkload.java class Javadocs for details on how to use it
with a client and workloads/tsworkload_template for parameters.
Also add the BasicTSDB instance for printing/debugging the workloads.