[CALCITE-4539] Customization of Metadata Handlers

[jamesstarr]

Changing RelMetadataQueryBase.map generic from Table<Object, List, Object> to Table<Object, Object, Object> to support more efficient cache keys.

[jacques-n]

@jamesstarr, can you rebase this on master?

[jcamachor]

@jamesstarr , what is the status of this PR? @jacques-n left some feedback but I think the PR was not synced to the main dev branch, it's not clear which issues are legit and need addressing, and which ones had already been solved.

[jamesstarr]

@jamesstarr , what is the status of this PR? @jacques-n left some feedback but I think the PR was not synced to the main dev branch, it's not clear which issues are legit and need addressing, and which ones had already been solved.

@jcamachor, @Jacques-n said he would look at it on tomorrow. Overall, I think I need to add some more javadocs and possibly clean up some naming, but I am waiting for feed back from @jacques-n before I do the last bit of clean up.