Removed 'using namespace process' from header files.

Review: https://reviews.apache.org/r/18249
Included stdint.h in stout files.

Review: https://reviews.apache.org/r/18446
Included stdint.h in libprocess files.

Review: https://reviews.apache.org/r/18447
Included stdint.h in mesos files.

Review: https://reviews.apache.org/r/18442
Increase upper bound for user time in SystemCpuUsage test.

Review: https://reviews.apache.org/r/18260