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.
issue