Add portable time abstraction (#738). Fixes #253.
From author's notes (@ben-foxmore):
The current usage of gettimeofday causes issues for us when performing
tests shortly after restarting a system. In our setup, this occurs
often as we restart the system before each test to ensure reliable
results. We already maintain our own version of iperf for some subtle
changes, but this change feels like it might be useful to upstream.
(It's also a reasonable size change, so we'd prefer not maintain it
with each new version of iperf.)
It uses clock_gettime on systems that have it available, and falls
back to gettimeofday when it's not. These two options use different
structures for storing time - clock_gettime uses timespec, and
gettimeofday uses timeval. To provide abstraction to which one is
available, a separate iperf_time struct is defined to store time.
timespec has nanosecond accuracy, while timeval only has microseconds.
For the purposes of iperf, I don't think nanosecond accuracy is
neccesary, so iperf_time only uses microseconds, throwing away any
additional accuracy. Currently I have used the MONOTONIC clock, as I
think we only need a consistent time interval measure.
