[dev] nsqd: add /topics endpoint

[stephensearles]

stephens2424: hey, did we not implement the /topics endpoint on nsqd with gossip?

mreiferson: I'd have to look
    I don't remember off the top of my head
    it seems like it would have to use gossip
    as the endpoint didn't exist prior to its introduction

stephens2424: oh for sure, but it looks like the endpoint isn't there. couldn't remember if we didn't do it for some reason other than just missing it

mreiferson: oh, hah
    probably just an oversight

stephens2424: yeah. looks like about a 12 line change
    jk: 11 lines: https://github.com/bitly/nsq/pull/571

mreiferson: hah
    do you think this endpoint naming is going to be confusing
    perhaps that's why we didn't add it
    i wonder if we should namespace these endpoints

stephens2424: maybe. the reason I want it right now is so nsq_to_file can just record all the topics on the one nsqd
    so whatever we do, if we're not 100% consistent with what lookupd has, let's at least make the utilities able to communicate with both

mreiferson: brb
    ahhh right
    nsq_to_file expects that endpoint
    I see
    are you ok with leaving that PR open for now so we can figure out the changes necessary to get this right?

stephens2424: that's fine with me