labelPair uses commutative comparison operator

labelPair should use non-commutative comparison operator so labelPair(a, b) is different from labelPair(b, a). Use e.g. edge if you do want commutation.

Attached a test program which gives different results in v1612 and develop.

Test-hashTable.C

This behaviour affects e.g. snappyHexMesh and lots of coupled boundary face matching.