Don't use complex datatypes as CAS tokens

For caches where CAS is not natively supported, we have a workaround, where the
CAS token is (based on) the stored value.

To confirm the data can be written to cache, the CAS token is compared against
"whatever is currently in cache", so we pull the cached data and rebuild the
value.

In the case of objects, we have now pulled & rebuilt (unserialized) 2 objects
that are actually the same object (for CAS purpose - it's the correct value to
overwrite), but in terms of ===, they're 2 different values.

This patch should make sure CAS tokens are always a serialized version of the
value we're saving (where no native CAS exists); these serialized versions can
reliably be compared.

Bug: 59941
Change-Id: I2760416c48f2ceb7a0e0c874dd70ec07b4dccdfc