[CORE] Rename: s/backends/backend

We have a couple of stuffs named with prefix `backends`. E.g.,

1. Maven module names (e.g., backends-velox)
2. Folder names (e.g., o/a/g/backendsapi)
3. Code (e.g. BackendsApiManager)

`backends` for code module is not likely a perfect name. Usually in code we use singular form to indicate a module that is an extension of a series of APIs, say `connector-`, `consumer-`, `engine-` etc. Plural is actually not usual as it could misleads one to treat it as "a set of something", though the actual intention was "one element in a set". Regarding word `backends` itself, it is a little bit difficult to pronounce comparing to `backend` either.

I'd propose to rename `backends` to `backend`. It could be a big change, so would be great to hear voices from the community.