storr::encode64() overhead

[wlandau]

#571 + profvis helped me see this.

See also richfitz/storr#44.

A few ideas:

1. A documentation-only fix: recommend a custom cache from storr::storr_rds(mangle_key = FALSE).
2. Use storr_rds(mangle_key = FALSE) by default for new caches and just mangle the file names.
3. Migrate to a specialized caching system for drake using times stamps and no key mangling.

[wlandau]

I think the best way to solve this is through richfitz/storr#88. We could use mangle_key = "selective" in new_cache() and is_default_cache() and handle back compatibility in darke_fetch_rds(). Simple on drake's end.

[wlandau]

If richfitz/storr#89 does not work out, we could change the default mangle_key to FALSE and use custom wrappers to mangle our own keys manually. As always, back compatibility is tricky.

[wlandau]

For the custom wrappers, make.names() would work fine for drake's purposes, and it is likely to be much faster than any key mangling done by storr.

[wlandau]

Hopefully we can handle this through richfitz/storr#90.

[wlandau]

Fixed via #633.

[wlandau]

See also #630.