stopwatch: add accessors for elapsed time, add a mode to collect all threads
stopwatch: add accessors for elapsed time, add a mode to collect all threads

This is useful from benchmarks which are multi-threaded, to measure the
total CPU usage of the benchmark.

Change-Id: Ic56191b52fa1eb80f37c42534c2a948b9792b20a
Reviewed-on: http://gerrit.ent.cloudera.com:8080/12
Tested-by: jenkins <c14f3aae4f03a600341a3b92ec1ffaac2338cde0@cloudera.com>
Reviewed-by: Matteo Bertozzi <1d5ca967930bde0bdce795acbf71a5e281d2d4e5@cloudera.com>

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