Possibility to have directory trees

...instead of huge flat directories, that is.

I was fearing that huge directories would have a negative effect on performance. This is not true, according to ogi in #1211166-16: Create separate directories for each cache bins. But now I already creted the code for this... so I submit it for review in a separate issue.

In my patch, set_encoding() sets various defaults for entity_cache_X bins, which may not be useful given the above, but I'm leaving it like this for now. The new directory level ogi mentions in #1211166-16: Create separate directories for each cache bins is possible with set_encoding(1).

This patch depends on #1211166-18: Create separate directories for each cache bins and #1488870-2: Reorganize cache expiry behavior being applied already. Those issues are way more important than this one.