test-priority: Unit test various configurations of candidate priorities
Change priority algorithm for better values

Now we will always give a handicap to UDP candidates, but still give
relays the lower priority. We have the following priorities now :
In reliable mode :
TCP Host : 120
TCP peer-refl : 110
TCP nat-assist : 105
TCP srv-refl : 100
UDP Host : 60
UDP peer-refl : 55
UDP nat-assist : 52
UDP srv-refl : 50
TCP relay : 10
UDP relay : 5

In unreliable mode :
UDP Host : 120
UDP peer-refl : 110
UDP nat-assist : 105
UDP srv-refl : 100
TCP Host : 60
TCP peer-refl : 55
TCP nat-assist : 52
TCP srv-refl : 50
UDP relay : 10
TCP relay : 5
Remove priority functions from public header file and move to agent-priv.h