fix(subscriber): return a promise on close

[scragg0x]

Fixes #212

Not sure if this is the best way to fix, but here you go!

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

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[codecov]

Codecov Report

Merging #223 into master will decrease coverage by 0.12%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
- Coverage     100%   99.87%   -0.13%     
==========================================
  Files          10       10              
  Lines         807      816       +9     
==========================================
+ Hits          807      815       +8     
- Misses          0        1       +1

Impacted Files	Coverage Δ
src/subscriber.js	99.37% <90.9%> (-0.63%)	⬇️
src/connection-pool.js	100% <0%> (ø)	⬆️

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[googlebot]

CLAs look good, thanks!

[scragg0x]

Sorry for the extra commits, probably want to squash if you merge.

[JustinBeckwith]

@callmehiphop can you take a look at this?

[callmehiphop]

@scragg0x sorry I kind of dropped the ball here, we actually have a handy util for promisifying our methods. If you want to take a crack at getting that working here, that would be amazing! Otherwise I can totally tackle this

[scragg0x]

@callmehiphop You can go ahead and take this. I didn't use the promisify lib since close calls 2 other async functions, one callback style and the other promise.