Polished skipping of cache_dirs inactive in a given strand (e.g. Coordinator)

Polished skipping of cache_dirs inactive in a given strand (e.g. Coordinator)
by adding SwapDir::active() method. The directory is active if it makes sense
to call its init/create/get methods in a given strand.

Fixed counting cache_dirs that need dedicated strands. We no longer assume
that all cache_dirs do but use SwapDir::needsDiskStrand() to ask each dir.
The result is stored in Config.cacheSwap.n_strands to optimize NumberOfKids().