Add some more javadoc for the PageCace APIs.
Reorganise and tidy up the page cache API a bit

Mostly by moving bits that are part of the StandardPageCache implementation, into the `standard` package.
Add package-info javadocs for the page cache API

Also notes a couple of TODOs for future test work that needs to be done,
and a change to the behaviour when running out of storage space.
Merge pull request #2853 from chrisvest/2.2-pagecache-docs

2.2 page cache javadoc improvements