Use digest256map for computing microdescriptor downloads
Use digest256_len in networkstatus_copy_old_consensus_info()

Now, if a router ever changes its microdescriptor, but the new
microdescriptor SHA256 hash has the same 160-bit prefix as the old
one, we treat it as a new microdescriptor when deciding whether to
copy status information.

(This function also is used to compare SHA1 digests of router
descriptors, but don't worry: the descriptor_digest field either holds
a SHA256 hash, or a SHA1 hash padded with 0 bytes.)
Merge remote-tracking branch 'public/bug13399'