Revprop packing implemented

Hi devs,

This week I had one of my "how hard can it be?" moments
and finally implemented revprop packing (did that mainly
offline). It passes all tests and seems to work pretty well.

It's design deviates from the existing revprop packing branch
in that it is more scalable and simpler to implement. Key
differences:

* Have a configurable limit (default 64k) to the pack file size.
  Concatenate revprops only up to that limit, but at least one
  revision's revprops per file. Have the manifest map revs to
  revprop pack files. IOW, let the OS do all the heavy lifting
  of storage management.
* Make revprop packing a mandatory part of FSFS packing
  in format 6 repos. There is no need to track revprop packing
  separately nor to give it a separate format info.

Since the new code will not be used unless you create
a format 6 repo, I'd like to commit everything directly to
/trunk for review instead of creating a new branch or
"overwriting" the existing one.

This is the order in which I want to commit the changes:

* refactor existing code
* update the design file
* add the revprop pack support
* add tests; write more tests
* bump the FSFS format

Any objections?

-- Stefan^2.

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