Add support for aged cache directories

[holdenk]

Eg on some servers there is X TB of NVMe and Y TB of SSD and K TB of HDD and it would be nice to have expiration just slowly move data down the stack in terms of speed.

[DiegoRBaquero]

I don't think this is configurable on nginx

[vorburger]

slowly move data down the stack in terms of speed

+1; this thought has crossed my mind as well.

I don't think this is configurable on nginx

It's probably just something we would have to "build" (as in, code out, with "domain knowledge"), not something Nginx could automagically "just do" for us with config.

[holdenk]

Yeah if theres interest in this maybe I'll take a crack it over the winter break, I've got a new server on with 8tb of NVMe and another 8tb of HDE I'd like to take advantage of (I know in practice right now the cache depth is likely to all fit inside the first 8tb of NVMe but you know the "future") :)

[vorburger]

@holdenk I suspect The Way To Go on this is probably https://man.archlinux.org/man/lvmcache.7 - WDYT?