cache::parse_key: supports_multiple_identifiers check a bit slow

The cache::parse_key function takes a fair chunk of time (in my test page it is called 482 times). As part of the MUC infrastructure this should be optimised.

I didn't find very much wrong with it, but a small detail is that the supports_multiple_identifiers check takes a significant amount of time. It is possible to optimise this (albeit with some duplication) by overriding the function in each of the cache store types for which it always has a fixed value anyway. (I left the mongodb cache alone, because that one's more complicated.)