bundle2: add a interrupt mechanism It is now possible to emit a single part in the middle of a payload production. This part will be processed with limitation (only access to a `ui` object). The goal is to let the server raise exception and output while a part is being processed. The source motivation is to transmit exception that occurs while generating a part. This change is was the motivation to bump the bundle2 format from HG2X to HG2Y. Somehow, the format bump made it into 3.2 without it. So this change go on stable. It is low risk as bundle2 is still disabled by default.

transactions: change backupfiles format to use newlines Previously the journal.backupfiles file was delimited by \0. Now we delimit it using \n (same as the journal file). This allows us to change the number of values in each line more easily, rather than relying on the count of \0's.

transactions: add version number to journal.backupfiles The transaction format will be changing a bit over the next releases, so let's go ahead and add a version number to make backwards compatibility easier. This whole file format was broken prior to 3.2 (see previous patch), so changing it now is pretty low risk.