nsqd: return error if pause state is not changed

[ploxiln]

so the caller of pause or unpause can know if the topic or channel
was already paused or unpaused

also avoids a little bit of work if no change

a humble idea, just to solicit opinions, could probably use better error strings

[mreiferson]

I guess it's useful to propagate this information?

[ploxiln]

Yeah I though it could be useful info to get in the response - sort of inspired by the corner-case of maybe having a script retry pause a few times until it takes effect. Maybe not worth it.

There's also the interesting bit where Pause() / UnPause() return type error, but (until this change) it is always nil. They could return the previous state as a bool instead. Or nothing. Dunno if any software out there really uses nsqd guts as a library, probably not, and we're breaking "api behavior" with the topic pause-or-start changes anyway ...

[mreiferson]

Ready to land this?

[ploxiln]

hang on, I can add a proper test pretty easily

[ploxiln]

I'm thinking about maybe returning 200 but with a body that says e.g. "already paused" somehow. May be less disruptive to existing scripts. Which may or may not even exist. I dunno.

Anyway I'd like to defer this PR until after #1053 is resolved.

[ploxiln]

I pushed up a "non-error" variant. But I'm thinking it's best to just close this until someone asks for this feature. (I don't really need it myself.)