Refactor and optimize `list()`

[ghost]

1) Use ES6 syntax to rewrite readMessages/unreadMessages instead of using index for a whole array. 2) Use Promise.all to wrap the mapping from read/unread messages and do Promise.all to await to save time.

• [x] npm test passes
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message follows commit guidelines

[codecov[bot]]

Codecov Report

Merging #139 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
+ Coverage   93.26%   93.27%   +<.01%     
==========================================
  Files          48       48              
  Lines        1664     1665       +1     
==========================================
+ Hits         1552     1553       +1     
  Misses        112      112

Impacted Files	Coverage Δ
app/controller/api/message.js	100% <100%> (ø)	:arrow_up:

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[ghost]

/cc: @JacksonTian