Support target throughput benchmarks and microseconds precision

[franz1981]

In order to allow fair comparisons of resource utilization and/or latencies between runs that push different load against a system, would be great to have a target throughput benchmark mode.
Given that sustainable throughput would produce tons of small latency values would be good to have
microseconds timestamp precision too (if the OS allow it).
#38 is addressing the former while https://github.com/franz1981/quiver/tree/micros_latencies need some refinement on python side to cover the latter.

[franz1981]

I have pushed franz1981@5860dc5 too, but beware that I'm really a dog in python :(
Running a target throughput bench with microseconds precision gives a very different picture of the latencies.
In milliseconds:

--------------------------------------------------------------------------------
Subject: activemq-artemis-jms q0 (/tmp/quiver-zss_ih7k)
Messages:                                       1,000,000 messages
Body size:                                            100 bytes
Credit window:                                      1,000 messages
Duration:                                            50.0 s
Sender rate:                                       20,003 messages/s
Receiver rate:                                     20,013 messages/s
End-to-end rate:                                   20,003 messages/s
Latency:
    0%:                                                 0 ms
   25%:                                                 0 ms
   50%:                                                 0 ms
   90%:                                                 1 ms
   99%:                                                29 ms
   99.9%:                                              66 ms
   99.99%:                                             70 ms
   99.999%:                                            72 ms
  100%:                                                72 ms
--------------------------------------------------------------------------------

While in microseconds:

--------------------------------------------------------------------------------
Subject: activemq-artemis-jms q0 (/tmp/quiver-jwaocfup)
Messages:                                       1,000,000 messages
Body size:                                            100 bytes
Credit window:                                      1,000 messages
Duration:                                            50.0 s
Sender rate:                                       20,004 messages/s
Receiver rate:                                     20,013 messages/s
End-to-end rate:                                   20,004 messages/s
Latency:
    0%:                                                42 us
   25%:                                                58 us
   50%:                                                65 us
   90%:                                               107 us
   99%:                                            20,199 us
   99.9%:                                          68,002 us
   99.99%:                                         79,679 us
   99.999%:                                        80,963 us
  100%:                                            80,978 us
--------------------------------------------------------------------------------