Wrap references to cache keys in `getKey`.
Wrap references to cache keys in `getKey`.

Created a function `getKey` that wraps references to the page cache. It
will allow for a transition of the value stored in the branch page cache
from the key to an object containing the key and the size of the key.

See #233.

bigeasy/strata,cesarmarinhorj/strata,bigeasy/strata,cesarmarinhorj/strata