bundle2: move bundle2 config option to section "experimental"
We highlight the fact that this is experimental by moving it to an "experimental"
section, and we match the config name with the server capability name
`bundle2-exp`.
bundle2: move all parts into a `bx2` namespace
All currently core parts are moved to a `bx2` namespace (for "bundle 2
experimental"). This should avoid conflicts between the final stable
format and the one about to be released.