After SERVER-72463, index keys can be generated for compound wildcard indexes. The format of the "multikey-path" index keys has changed to the compound pattern. We should extend helpers in 

src/mongo/db/query/wildcard_multikey_paths.h

to support compound wildcard indexes.

Currently all the helpers in the file still assume that the key pattern is a single-field wildcard index.  We should lift all relevant invariants and create a correct IndexBounds to query only the multikey path keys in a CWI.