osd: set default cache_target_{dirty,full}_ratios based on configurable

These were hard-coded in the pg_pool_t constructor, but that was a dumb
idea.

Note that decoding legacy pg_pool_t's no longer does what it used to.  I'm
pretty sure that's okay since we care less about interim releases and
because we are pulling these normally out of OSDMap, which is freshly
encoded on a regular basis (and certainly recently with real values). Also,
let's not forget that this field is meaningless on old pools anyway.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: add tunables for cache_min_{flush,evict}_age

Why not.

Signed-off-by: Sage Weil <sage@inktank.com>