The expected behavior is when timing is True, then we record each
request's time, then we should call reset_timings() to release memory.
However, the shell, client.{HTTPClient,SessionClient} will record each
request's time no matter what timing is set, then after long running
time in service like ceilometer-agent-compute, the memory keeps increasing.
We'd better not record request's time when timing is set to False.
Users are not responiable to call reset_timings() when they don't want
timing functionality.
Change-Id: I3e7d2fadf9a21be018781d528a1b6562228da6dd
Closes-Bug: #1433491