cells: Minor cleaning of CellPath and CellAddressCore

Computes better hashes. Reduces cost of reverting CellPath.

The precomputed hash in CellPath is kept for backwards compatibility.
It can be dropped in 2.11.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7028/