bundle2: move handler validation out of processpart
As part of refactoring bundle part processing let's move handler validation to
its own function.
Differential Revision: https://phab.mercurial-scm.org/D707
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