verify associated files when checking numcopies

Most of this is just refactoring. But, handleDropsFrom
did not verify that associated files from the keys db were still
accurate, and has now been fixed to.

A minor improvement to this would be to avoid calling catKeyFile
twice on the same file, when getting the numcopies and mincopies value,
in the common case where the same file has the highest value for both.
But, it avoids checking every associated file, so it will scale well to
lots of dups already.

Sponsored-by: Kevin Mueller on Patreon

doc/todo/numcopies_check_other_files_using_same_key.mdwn

@@ -15,3 +15,10 @@ differently than in a non-bare repo. (Also if this is done, the preferred
 content checking should also behave the same way.) The docs for --all 
 do say that it bypasses checking .gitattributes numcopies.
 --[[Joey]]
+
+> Note that the assistant and git-annex sync already check numcopies
+> for all known associated files, so already handled this for unlocked
+> files. With the recent change to also track
+> associated files for locked files, they also handle it for those.
+> 
+> But, git-annex drop/move/import don't yet.