bundle2: move processpart stream maintenance into part iterator The processpart function also did some stream maintenance, so let's move it to the part iterator as well, as part of moving all part iteration logic into the class. There is one place processpart is called outside of the normal loop, so we manually handle the seek there. The now-empty try/finally will be removed in a later patch, for ease of review. Differential Revision: https://phab.mercurial-scm.org/D706