bundle2: allow bundle2 for pulling over the wire This changeset makes `wireprotocol` peers advertise bundle2 capability and comply with bundle2 `getbundle` requests. Note that advertising bundle2 could make a client try to use it for push. Such pushes would fail. However, I do not expect any human being to have enabled bundle2 on their server yet.

bundle2: return a stream from exchange.getbundle For friendliness with the wire protocol implementation, the `exchange.getbundle` now returns a binary stream when asked for a bundle2. We detect a bundle2 request and upgrade the binary stream to an unbundler object. In the future the unbundler may gain feature to look like a binary stream, but we are not quite there yet.

bundle2: let readbundle return unbundle20 The `readbundle` function can now recognize a bundle2 stream and return the appropriate unbundler. This is required for proper bundle2 support over the wire.

bundle2: make header reading optional The `readbundle` function will consume the 4 first bytes to dispatch between various unbundler. We introduce a way to inform `unbundle20` that the header has been read and it can be trusted.

bundle2: prepare readbundle to return more that one type of bundle We first read 4 bytes to get the `HG10` bytes then we read the compression scheme if this is `HG10`. This prepares the code for the arrival of `HG20` handling.

bundle2: add a ui argument to readbundle The bundle2 unbundler needs a ui argument. We are now passing this information to `readbundle`.

bundle2: move `readbundle` into the `exchange` module The `readbundle` function is going to understand the bundle2 header. We move the function to a more suitable place before making any other changes.

bundle2: use headerless HG10UN stream in changegroup Using `readbundle` in the part handlers creates a circular import hell. We are now using a simple `HG10UN` stream with no header. Some parameters may later be introduced on the part to change parameter. Producers are updated as well.

bundle2: allow using bundle2 for push We now support bundle2 for local push. The unbundle function has to detect which version of the bundle to use since the return type is different. Note that push error handling is currently nonexistent. This is one of the reasons why bundle2 is still disabled by default.

bundle2: add a "check:heads" handler This part is intended to hold the same role as the `heads` argument of the unbundle function. The client fill it with the known heads at bundle time and the server will abort if its heads changed.