Implement major FSFS performance related changes in the experimental FSX format

I propose to design and implement all major performance related
changes of the current FSFS format in the experimental FSX format.
I mean features exactly like revprop caching and log addressing.

The best possible option for such features is to be implemented and
released as a part of experimental FSX. Then, when we will be really
sure that everything is fine and it works for the wide number of users,
we can selectively port (not merge) some of the features into the FSFS.

We have successfully followed this approach in the past (FSFS itself
and ra_serf) and currently I do not see any reasons to change this
approach for features I'm talking about. Moreover, we have discussed
this on Berlin 2013 hackathon [1]:
[[[
stefan2 expressed that while he is confident that FSFSv7 is solid code,
it's also quite critical and could easily take a year or more to fully
stabilize. Attendees felt that perhaps it would be best to introduce FSFSv7
as a new, experimental fs-type. Stefan said he had been thinking
about the same thing himself, even considering a different name for
his implementation.
]]]

At this point I'm '-1' to:
1) release the improved version of revprop caching to the FSFS format [2]

2) release the log addressing feature in the FSFSv7 format

The current implementation of revprop caching and log-addressing features
should be reverted from trunk and moved to FSX.

[1] http://wiki.apache.org/subversion/Berlin2013#FSFSv7_Branch_Reintegration
[2] http://svn.apache.org/repos/asf/subversion/branches/revprop-caching-ng

-- 
Ivan Zhakov