Add freqoff_to_period() function that converts frequency in the
form of f = f0 + ax, where f0 is a central frequency, a is some
scaling constant and x is desired offset into a corresponding period.
o Get rid of stable.target_runtime and replace it with
stable.target_pfreq, which is more natural gauge to use (i.e.
frequency, not period).

o use  the new freqoff_to_period() function to scale our loop
delay around central target frequency. This plugs nicely
into the PLL framework, replacing hand-crafted function to make
delay/frequncy follow loop filter.

o get rid of use (proliferation?) of pre-defined POLL_RATE, use
new dynamic target_pfreq instead;

o make shced_hz an upper limit on target_pfreq, so we don't try
to run 200 HZ loop with just 100 HZ scheduler.
Merge branch 'rtpp_2_0' of github.com:sippy/rtpproxy into rtpp_2_0