Refactor storing of measured_bw versus Unmeasured=1.

This patch moves the measured_bw field and the has_measured_bw field
into vote_routerstatus_t, since only votes have 'Measured=XX' set on
their weight line.

I also added a new bw_is_unmeasured flag to routerstatus_t to
represent the Unmeasured=1 flag on a w line.  Previously, I was using
has_measured_bw for this, which was quite incorrect: has_measured_bw
means that the measured_bw field is set, and it's probably a mistake
to have it serve double duty as meaning that 'baandwidth' represents a
measured value.

While making this change,I also found a harmless but stupid bug in
dirserv_read_measured_bandwidths: It assumes that it's getting a
smartlist of routerstatus_t, when really it's getting a smartlist of
vote_routerstatus_t.  C's struct layout rules mean that we could never
actually get an error because of that, but it's still quite incorrect.
I fixed that, and in the process needed to add two more sorting and
searching helpers.

Finally, I made the Unmeasured=1 flag get parsed.  We don't use it for
anything yet, but someday we might.

This isn't complete yet -- the new 2286 unit test doesn't build.