Teleport tubes forbid other uses of mod storage

[Watilin]

Currently, teleport.lua#L106 throws an error when the storage contains any key that isn't recognized as teleport tube stuff. It prevents other uses of mod storage, for example in #175 (instead of file storage).

I suggest we modify teleport functions to prefix their storage keys, and only handle keys having the relevant prefix. I'm willing to do it. I know that a tptube API is exposed, but after a quick look at the code, i think it's possible to do the change transparently and stay with the current API version number.

[SwissalpS]

Edit: nvm, that's not how mod storage works.

I'm wondering why all that string manipulation is used. While the db was stored in a flat file it made sense but now the tube-db-table could just be passed to mod_storage directly. Smth like:

local storage_fields = {
    version = tube_db_version,
    tp_db = tube_db,
    mesecon_detector_db = mesecons_db
}
storage:from_table({ fields = storage_fields })

and thus let the backend deal with serializing.