Slow down on throttling / max-age

[lemon24]

Slow down based on 429 Retry-After and Cache-Control max-age.

OK, Retry-After support done directly in the updater; bumping the interval should probably be done as part of a plugin (feels somewhat icky to do it in core); also, the plugin may do other HTTP things, like #246.

Originally posted by @lemon24 in #307

To rethink if this should be a plugin or a core feature:

• ideally, we should store statuses and headers for the last X update attempts (easier to do in core, should happen by default)
• it should be the default and hard to disable by mistake
• for a hosted version of reader, it should not be possible to users to bypass it (so it can't rely on tags, or alternatively, we don't allow modifying that tag)

Related: Cache-Control handling #376

[lemon24]

See why "we should store statuses and headers for the last X update attempts" is probably not a good idea: #404 (comment)

[lemon24]

For hosted reader, the shared update mechanism needs to know if a feed had Cache-Control: private on the last update, so whatever we do to store headers, it should support this as well.