Convert to Promise-based `fs` methods

[groenroos]

Convert from Sync methods to Promise-based methods to improve server performance.

Did not do it for test suites, as blocking is less important there. There is also a check in Uploads that needs refactoring, but no clear way how to do that.

Closes #432.

[codecov[bot]]

Codecov Report

Merging #435 (65c0f96) into master (798f4e3) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #435      +/-   ##
==========================================
+ Coverage   98.02%   98.07%   +0.04%     
==========================================
  Files          37       37              
  Lines        4607     4668      +61     
==========================================
+ Hits         4516     4578      +62     
+ Misses         91       90       -1     

Impacted Files	Coverage Δ
core/loadConfig.js	100.00% <100.00%> (ø)
core/loadController.js	100.00% <100.00%> (ø)
core/loadHooks.js	98.85% <100.00%> (ø)
core/loadModel.js	100.00% <100.00%> (ø)
core/loadPermissions.js	100.00% <100.00%> (ø)
drivers/render/Html.js	100.00% <100.00%> (ø)
lib/Utils.js	100.00% <100.00%> (ø)
lib/Uploads.js	98.78% <0.00%> (+0.21%)	:arrow_up:
lib/Templating.js	99.07% <0.00%> (+0.92%)	:arrow_up:

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