Move volatile-cache to workerd

[jasnell]

First steps to moving volatile-cache to workerd. This will remain draft until the tests are also ported and the internal PR switching over to this is opened.

This is effectively a straightforward port of the version in the internal repo with a callback hook replacing some of the internal-specific additional limits we enforce when resizing.

Key questions to answer:

• Should we rename the API before it lands here? What should the name be? Renamed to MemoryCache (at least for now)
• Should any changes be made to the API before it lands here? What changes?
  • getInstance(...) method changed to also accept the ownerId
  • virtual interface added to abstract the actual implementation of the cache map
• Should any changes be made to the binding configuration before it lands here? What changes?
  • id field is now optional
• Should the implementation be updated to use memcache now or later?

[jasnell]

Do we want to mark this experimental initially until we're ready to enable this in production or is landing it without the experimental flag ok here?

[irvinebroque]

Really seems like needs to be experimental flag.

[jasnell]

Given that this is renamed, it will no longer conflict with VolatileCache in the internal repo and shouldn't need to wait for the internal repo to be updated to land.

[jasnell]

Updated in 1ee1e0f