Add CPU Freq to ETS

It would be useful in debugging a range of issues if we had historical cpu frequency (per core) in the ETS tables.   I believe Sigar already captures the current frequency but will need testing.  http://src.couchbase.org/source/xref/5.0.0/sigar/include/sigar.h#141

Would be helpful in situations where a customer sees performance issues or timeouts when the CPU clock changes due to Power Management states or if the machine is a VM and gets speed adjusted for events like live VM migration aka vMotion.