Rename option cacheExpiry -> experimentalCacheExpiry

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• tests
• documentation
• metadata

Breaking Changes?

• yes (but only if we release current master before merging this)
• no

If yes, please describe the breakage.
Option renamed.

Please Describe Your Changes

I would prefer to make the new cacheExpiry option an experimental option experimentalCacheExpiry until we had some time to play around with this and are sure this is the way we want to configure cache expiry. Rationale:

• Being "experimental" signals this is in development and gives us more freedom to change how it works
• We can safely document this as an experimental option so documentation is not forgotten again but it is not "set in stone"
• Instead of counting runs, maybe it turns out measuring memory consumption or a combination of both is a better fit? Or it turns out this fine-grained control is not necessary and only very few values provided via a high-level interface work better here?

This PR also fixes some styling which shows me it is a good idea to enforce linting on the client side.

[guybedford]

Sure.

[shellscape]

Sounds reasonable to me

[shellscape]

Gah, sorry wrong radio button.