HTTPv2 introduces performance regression in certain ra_serf operations

As reported on the dev@ list[1], Subversion's trunk code appears to introduce a
performance degradation when using 'svnsync' and svn_ra_serf against an
HTTP-v2-enabled server to sync revisions from a remote server to a local
(file://) repository.  As HTTPv2 was intended to only reduce the number of
requests required to fulfill and operation -- not to add new requests or change
the size of the payload of existing requests/responses -- this performance
regression is completely unexpected.

[1] http://svn.haxx.se/dev/archive-2010-05/0142.shtml