[atomic-revprops] status update

For the last few weeks I've been working on the atomic-revprop branch.

(Its goal is port the new svn_fs_change_rev_prop2() to libsvn_ra, which
should allow callers to specify both a value and an optional "previous
value" and have the revprop change atomic; see [1].)

Currently, the API is implemented over all RA layers, and the test (prop 34)
passes.

To extend svnsync to use the new API for acquiring locks,

[1] http://thread.gmane.org/gmane.comp.version-control.subversion.devel/120330/focus=120416