Unmap the microdescriptor cache before replacing it.

This is a reprise of the fix in bdff7e32; 6905c1f6 reintroduced
that bug.  Briefly: windows doesn't seem to like deleting a mapped
file.  I tried adding the PROT_SHARED_DELETE flag to the createfile
all, but that didn't actually fix this issue.  Fortunately, the unit
test I added in 4f4fc63f should
prevent us from making this particular screw-up again.

This patch also tries to limit the crash potential of a failure to
write by a little bit, although it could do a better job of retaining
microdescriptor bodies.

Fix for bug 8822, bugfix on 0.2.4.12-alpha.