This should ensure that we have enough shortly after startup for initial
path builds.
The spread speed here gets slightly increased to lifetime/5 (=4min)
instead of lifetime/4 (=5min) so that our "normal" number of paths is 5
with occassional momentary drops to 4, but should always keep us >= the
new minimum of 4.
Because the path spread happens over time, this shouldn't result in a
rebuild of several paths: we'll build 4 quickly, then another at +4m,
another at +8m, etc. When the initial 4 expire, we'll be dropping from
9 to 5 established but that's still above the minimum (4) so we won't
need to reconnect to several at once, and the spread builds should keep
us at 5 all the time.
These aren't needed: CMake already knows how to follow #includes and
rebuild when headers change as long as the headers are included
*somewhere*. The extra .cpp files here just require building a bunch of
.cpp files with just header content that we just end up throw away
during linking (since the same things will also be compiled in whatever
other compilation units include the same headers).
This rewrites the version info using lokid's approach of compiling it
into a .cpp file that gets generated as part of the build (*not* during
the configure stage).
Among other things, this means that changing the version no longer
invalidates ccache or cmake dependencies, and because it depends on
`.git/index` git commits will cause the version to be regenerated,
making the commit tag more reliable (currently if you rebuild without
running cmake your git commit tag doesn't update).
