Add availability calculation

This patch enables ssbench to show availability.
This will enable each ssbench-worker to report actual
request count which included retries.
After that, ssbench-master will calculate availability
from succeeded request count and actual request count.
Currently, only succeeded result includes actual request count.
(Exception results are ignored in availability calculation.)

As a result, ssbench shows ActualRequestCount and Availability
as follows.

'''
  TOTAL
         Count:   613  Average requests per second:  79.8
+        ActualRequestCount(including retries):    613  Availability: 1.000
'''

ToDo:
Includes Exception results in order to calculate availability
correctly.