Remove scales dependency with self-contained metrics implementation

The scales library had its last release in 2015 and was only tested on
Python 2.7/3.3. This change replaces it with a self-contained metrics
implementation that provides the same functionality:

- IntStat: Thread-safe integer counter
- PmfStat: Percentile/distribution statistics with reservoir sampling
- Stat: Gauge statistics with callable evaluation
- StatsCollection: Named collections of statistics
- Global registry for metrics access via getStats()

The new implementation maintains API compatibility with the existing
metrics interface used by the driver.

Fixes #665

Author: dkropachev

benchmarks/base.py

@@ -21,7 +21,7 @@
 from optparse import OptionParser
 import uuid
 
-from greplin import scales
+from cassandra.metrics import getStats
 
 dirname = os.path.dirname(os.path.abspath(__file__))
 sys.path.append(dirname)
@@ -192,7 +192,7 @@ def benchmark(thread_class):
         log.info("Total time: %0.2fs" % total)
         log.info("Average throughput: %0.2f/sec" % (options.num_ops / total))
         if options.enable_metrics:
-            stats = scales.getStats()['cassandra']
+            stats = getStats()['cassandra']
             log.info("Connection errors: %d", stats['connection_errors'])
             log.info("Write timeouts: %d", stats['write_timeouts'])
             log.info("Read timeouts: %d", stats['read_timeouts'])