bundle2: allow pulling changegroups using bundle2 This changeset refactors the pull code to use a bundle2 when available. We keep bundle2 disabled by default. The current code is not ready for prime time. Ultimately we'll want to unify the API of `bunde10` and `bundle20` to have less different code. But for now, testing the bundle2 exchange flow is an higher priority.

bundle2: add an exchange.getbundle function This function can return a `HG10` or `HG20` bundle. It uses the `bundlecaps` parameters to decides which one to return. This is a distinct function from `changegroup.getbundle` for two reasons. First the APIs of `bundle10` and `bundle20` are not compatible yet. The two functions may be reunited in the future. Second `exchange.getbundle` will grow parameters for all kinds of data (phases, obsmarkers, ...) so it's better to keep the changegroup generation in its own function for now. This function will be used it in the next changesets.

localpeer: propagate bundlecaps in getbundle call Best arguments are the ones in use...

bundle2: make it possible have a global transaction for the unbundling We use the `gettransaction` method approach already used for pull. We need this because we do not know beforehand if the bundle needs a transaction to be created. And (1) we do not want to create a transaction for nothing. (2) Some bundle2 bundles may be read-only and do not require any lock or transaction to be held.