Client workload generator

Start making the client workloads more robust.

Overview of this change:

• Cleaned up src/client.cpp
• Multiple workloads can run efficiently from the same workload engine. These are defined in src/clockwork/workload/workload.h and .cpp
• Specific workloads used for specific experiments can be defined in src/clockwork/workload/example.h or in other files
• src/client.cpp lets you specify which workload to run.

Current supported workload types are:

• ClosedLoop: runs concurrency requests in closed-loop
• BurstyClosedLoop: like ClosedLoop, but defined by a burst and idle period. The durations of burst and idle are drawn from poisson distributions, and the BurstyClosedLoop workload is paramaterized by the mean burst and mean idle duration.
• FixedRate: submits requests at a fixed rate. Paramaterized by the rate
• PoissonOpenLoop: like FixedRate, but exact request inter-departure times are drawn from a poisson distribution
• BurstyPoissonOpenLoop: like PoissonOpenLoop, with burst and idle periods similar to BurstyClosedLoop.

Other workloads to add:

• Trace-replay workloads (e.g. from the Azure traces)