HashBagOStuff: use the value itself as the CAS token

Avoid an unnecessary serialize() call on get() requests, which makes it
faster and also happens to work around a nasty crash bug which is
causing lots of segfaults right now.

CAS semantics should be the same, if the caller handles a CAS token in
an opaque way and does not assume it is a string. I could only find one
caller of cas() in core and WMF deployed extensions, and that is
BagOStuff::mergeViaCas(), which does correctly handle CAS tokens of any
type.

Bug: 71724
Change-Id: I0b0b5f0157c005ec7f76210dfb8576c056314733