Duration related refactoring changes.

- Fixed Duration's stream insertion operator so that the output string
  does not have trailing zeros after the decimal point.

- Changed Duration to 1) use int64_t to store the value in nanoseconds
  for better precision; 2) use integers in constructors; 3) include
  overloaded multiplication & division operators.

- Try<Duration> Duration::create(double seconds) is added to work with
  a floating-point seconds input.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/11202
Time related refactoring changes.

- Added a Time class as a new abstraction that works with Duration for
  various time related calculations.

- Added a Try<Time> Time::create(double secs) corresponding to the
  Duration::create() API.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/11283
Duration-Time related refactoring changes.

- Changed various old *time* and *duration* related variables from
  "double", "Seconds" type to use "Duration" and "Time" type.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/11285
Performed GTEST_IS_THREADSAFE check.

Review: https://reviews.apache.org/r/11264
Removed unnecessary use of Option.

Review: https://reviews.apache.org/r/11265
Refactored zookeeper_tests.cpp into master_detector_tests.cpp and
group_tests.cpp.

Review: https://reviews.apache.org/r/11266
Made tests flags inherit from logging.

Review: https://reviews.apache.org/r/11267
Moved 'tests::mkdtemp' to Environment.

Review: https://reviews.apache.org/r/11268
Minor cleanup in stout/exit.hpp.

Review: https://reviews.apache.org/r/11269
Improved library for using JVM/JNI and updated uses (in tests).

Review: https://reviews.apache.org/r/11270
Removed unused variables in AllocatorZooKeeperTest.

Review: https://reviews.apache.org/r/11271
Renamed tests/zookeeper_test.hpp|cpp to zookeeper.hpp|cpp.

Review: https://reviews.apache.org/r/11272
Refactored MesosTest/MesosClusterTest into a generic fixture for
launching in-memory Mesos clusters and updated all tests
appropriately.

Review: https://reviews.apache.org/r/11273
Cleaned up the output from running and automagically disabling the
CgroupsNoHierarchyTest.

Review: https://reviews.apache.org/r/11274
Used Milliseconds rather than Duration::parse.

Review: https://reviews.apache.org/r/11275
Removed unnecessary TestingIsolators.

Review: https://reviews.apache.org/r/11276
A little spring cleaning in the allocator tests.

Review: https://reviews.apache.org/r/11278
More spring cleaning, this time in the slave recovery tets.

Review: https://reviews.apache.org/r/11279
Replaced local::launch in tests with MesosTest.

Review: https://reviews.apache.org/r/11280
Removed unused local::launch overload.

Review: https://reviews.apache.org/r/11281