fix LPM extraction for composite keys

[nicolaskagami]

• LPM extraction now checks the size of the type instead of the size of the key, which was only correct if the key wasn't composite.
• It now correctly considers the "length" byte when advancing the offset.
• A test was added to exercise those two scenarios with a composite (lpm, exact) key.

[rcgoodfellow]

Mostly LGTM. Has this been tested in voxel, a4x2 or omicron-local?

[nicolaskagami]

Mostly LGTM. Has this been tested in voxel, a4x2 or omicron-local?

This was a required change for my voxel runs for the 662 poc, though they were run in a heavily altered system. I'll do a clean run with just this and the other p4 fixes.

[nicolaskagami]

Mostly LGTM. Has this been tested in voxel, a4x2 or omicron-local?

Successfully ran voxel (rss, commtest) with this and the other two P4 changes.