Fix uncaught error in async middleware

[hoangvvo]

I turned loop into an async function to allow it to be caught in next. I attempted to test out the performance implication by this change to async. This was run on node v12.16.3 and shows no significant difference but there may be for older versions that I did not get to.

hey -z 10s -m GET http://localhost:3000/user/123
// run 3 times, take the last one

this pr:

Summary:
  Total:	10.0018 secs
  Slowest:	0.0122 secs
  Fastest:	0.0001 secs
  Average:	0.0016 secs
  Requests/sec:	30993.6104
  
  Total data:	2789919 bytes
  Size/request:	9 bytes

polka/next:

Summary:
  Total:	10.0018 secs
  Slowest:	0.0116 secs
  Fastest:	0.0001 secs
  Average:	0.0016 secs
  Requests/sec:	30834.5827
  
  Total data:	2775618 bytes
  Size/request:	9 bytes

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Closes #134
Closes #164

[kinsi55]

How many middlewares / handlers did you have when testing? Surely having like 5-10 would cause a significant difference

[hoangvvo]

How many middlewares / handlers did you have when testing? Surely having like 5-10 would cause a significant difference

I used the one from bench/polka. Still, I think it will only be when there are about somewhat >50 or >30 that there is a significant diff.

[codecov-commenter]

Codecov Report

Merging #135 (2d3dbc1) into next (66f9cc5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              next      #135   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          108       105    -3     
=========================================
- Hits           108       105    -3     

Impacted Files	Coverage Δ
packages/polka/index.js	100.00% <100.00%> (ø)

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[lukeed]

Thank you @hoangvvo! Sorry for the (massive) delay on this.

Confirmed that on my machine, at least with Node 14.15.3, that both this PR and the current next run at 37.2k and 37.3k req/sec.

This does require Node 8.x, which is technically breaking, but that's what next is for :)