enhancement: multiple block stores with usage policies

#### Type: Enhancement
#### Severity: Medium
#### Description:

If I have a very fast SSD and a very slow magnetic archive disk, I'd like some way to direct the ipfs daemon to use them according to some flags I can set through `ipfs config` and `ipfs pin`.

First, there would need to be a way to designate different block stores in separate directories, leaving it to the user to mount whatever disks there appropriately. In the config, each store could be marked with a relative performance value (positive integer; ∈ Z-+).

Second, when setting pins, there could be a similar `--availability` option to assign a similar value to pinned objects. The initial `ipfs add` would try to use a store that had a performance value at least as high as the availability value. When garbage collecting, objects could be rearranged in a way that prioritized storing higher-availability objects on faster storage, including: swapping out pinned objects that have had that value lowered; and accounting for storage limits that have changed or been hit, reclaiming more performant space as necessary.