[master-test] updated GetTableSchema() microbenchmark
[master-test] updated GetTableSchema() microbenchmark

Updated GetTableSchema() RPC benchmark:
  * added histogram for the PRC latency
  * use multiple proxy to originate concurrent requests

The updated results look like the following for a RELEASE build
(microsecond is the unit for the histograms' values):

  GetTableSchema RPC: 105427.4 req/sec (authz disabled)
  Count: 527151
  Mean: 12.3581
  Percentiles:
     0%  (min) = 3
    25%        = 7
    50%  (med) = 9
    75%        = 14
    95%        = 21
    99%        = 65
    99.9%      = 231
    99.99%     = 406
    100% (max) = 7790

  GetTableSchema RPC: 39110.2 req/sec (authz enabled)
  Count: 195551
  Mean: 249.154
  Percentiles:
     0%  (min) = 56
    25%        = 72
    50%  (med) = 199
    75%        = 334
    95%        = 624
    99%        = 772
    99.9%      = 1176
    99.99%     = 2080
    100% (max) = 4172

Change-Id: Ie49fd45f86b258ae4e441a3d3f272b5f52019169
Reviewed-on: http://gerrit.cloudera.org:8080/15894
Reviewed-by: Andrew Wong <b68e4fdc6430321a6b47400732ff97d7ae91234e@cloudera.com>
Tested-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

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