linked_list-test: dump a histogram of insert performance upon completion
linked_list-test: dump a histogram of insert performance upon completion

This captures a histogram of the latency of inserts as seen by the client
during the integration test. The histogram contents are dumped periodically
and then finally at the very end of the test run.

This can be useful for comparing latency performance from build to build.

Example output:
  I0529 14:45:30.161455 15791 linked_list-test.cc:233] Written 500000 rows in chain
  ------------------------------------------------------------
  Histogram for latency of insert operations (microseconds)
  Note: each insert is a batch of 50 rows.
  ------------------------------------------------------------
  Count: 10000
  Mean: 235.662
  Percentiles:
     0%  (min) = 149
    25%        = 175
    50%  (med) = 186
    75%        = 207
    95%        = 542
    99%        = 892
    99.9%      = 3086
    99.99%     = 5220
    100% (max) = 7323

The last printout additionally includes the gflags.

Change-Id: I9c2e9f99bce198d7a6ea0723e5e032558bcb9a43
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2728
Reviewed-by: Jean-Daniel Cryans <4bf4c125525b8623ac45dfd7774cbf531df19085@cloudera.com>
Tested-by: jenkins

cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu