Improve resolver cache lock and thread synchronization

This change comprises:
1. replace pthread_mutex with std::mutex to realize RAII.
2. have a single condition variable to prevent race condition between
   threads.
3. add 'predicate' to avoid spurious awakenings.
4. add a parameter in GetResolverInfo API which enabling test cases to
   know if timeout happened among concurrent DNS queries. Also, dump it in
   bugreport.
5. verify if test case GetAddrInfoV6_concurrent,
   GetAddrInfoStressTest_Binder_100 and
   GetAddrInfoStressTest_Binder_100000 pass because of timeout on
   concurrent DNS queries.

Bug: 120794954
Test: runtests.sh pass
Test: run ResolverTest.GetAddrInfoV6_concurrent 100 times
Test: run ResolverTest.GetAddrInfoStressTest_Binder_100 100 times
Test: run ResolverTest.GetAddrInfoStressTest_Binder_100000 100 times
The test script:
----------------------------
for i in $(seq 1 100)
do
  adb shell resolv_integration_test --gtest_filter=ResolverTest.GetAddrInfoV6_concurrent
  adb shell resolv_integration_test --gtest_filter=ResolverTest.GetAddrInfoStressTest_Binder_100
  adb shell resolv_integration_test --gtest_filter=ResolverTest.GetAddrInfoStressTest_Binder_100000
done
exit 0

Change-Id: I4bdc394ba7ded7a6b7239f2d35b559a4262cb7b9