Two important fixes to CPR decoding: wrong cast and more precise timing.

This commit address two issues with the implementation of CPR decoding:

Fix #1: The two functions N() and DLon() used to have the latitude
parameter as an integer (!), basically truncating the fractional part
before calling the NL() function to perform the lookup.

This resulted into random strange movements of aircrafts, especially
jumps or shifted positions.

Fix #2: Use milliseconds for CPR odd/even timestamps. Dump1090 already
tried to use the most recent packet received among the odd and even
packets currently available, however to do this correctly millisecond
resolution should be used, otherwise many times the odd and even packet
appear to have the same time in seconds and we don't always use the
latest received.