have timeout for cache

[bsipocz]

Would be nice to have a mechanism to invalidate the cache. E.g. when new data is added at the serverside users should be able to automatically get access to that. Whether to achieve that with a timeout (which wouldn't work well right around when the new data is added, and would ignore cache unnecessary when there is no data added for a long time) or my some other means is up for discussion.

[bsipocz]

A caching issue came up again on a twitter thread, this time regarding the jplhorizon module (where ephemeris got updated server side, yet the cached version got used in the query).

I marked both cache related issues (#1511) as critical for the next release, and I feel this should be resolved sooner rather than later.

[keflavich]

We have yet another affected module.

Much as I love caching, maybe we need to start defaulting it to 'off' to circumvent these issues?

[bsipocz]

@ceb8 has resurrected the old cache timeout PR in #1634, I'll try to review what she has there soonish instead of changing the defaults.

[bsipocz]

#1634 meant to address this, reopen if it's not the case.