Combine "m" lines with identical hash values

dirserv_generate_networkstatus_vote_obj() recently (or soon to be, depending on how you see it -- it sits in my branch bug6363_5535) started generating two "m" lines in votes, one for consensus methods 8-13 and one for method 14 ("a" lines).

For relays without an IPv6 address (i.e. not including an "or-address" line in their descriptor), the two hashes will be identical.

As an optimisation we could combine two "m" lines with identical hash values. As an example, instead of saying

    "m 1,2,3 H1"
    "m 4,5 H1"

we would say

    "m 1,2,3,4,5 H1"

but still say

    "m 1,2,3 H1"
    "m 4,5 H2"

This affects the size of the votes only and don't affect bandwidth consumption for ordinary relays or clients in any way.