Fix several remotely triggerable asserts based on DirPort requests

for a v2 or v3 networkstatus object before we were prepared. This
was particularly bad for 0.2.0.13 and later bridge relays, who
would never have a v2 networkstatus and would thus always crash
when used. Bugfixes on 0.2.0.x.

Estimate the v3 networkstatus size more accurately, rather than
estimating it at zero bytes and giving it artificially high priority 
compared to other directory requests. Bugfix on 0.2.0.x.


svn:r12952