OLCTriangle: use a mixed breadth-first/depth-first search for large trees

This won't affect fast problems but can reduce the memory usage for hard
ones.

Experiments with worst-case flights with many gps fixes revealed a
saving of over 2/3 compared to the old strategy (450mb instead of 1.6gb
for a flight with 4096 points). Flights with less points don't have that
problem, due to a nonlinear dependency to flight size.
OLCTriangle: reduce memory footprint of CandidateSet

Saves about 10% memory...
Driver/LX: support Nano firmware 2.10

The Nano protocol was changed after firmware 2.05.  This is some more
fallout of TRAC #2819.
Android/InternalGPS: detect when GPS is lost on Android 2.3 and older

Fix for TRAC #2929.
Driver/FLARM: verify checksums of answers

This attempts to improve the reliability of task declarations.
release v6.6.3
Merge tag 'v6.6.3' into v6.6.x

Conflicts:
	NEWS.txt