Daemon optimizations

[palourde]

Description

This PR introduces a couple of changes around the fetcher, in order to optimize the operations and prevent data races:

• Prevent a data race around the APIs list shuffling
• Prevent data races when accessing the "snapshot" data of a datacenter
• Use a RWMutex so the API can request multiple read locks simultaneously
• Refactoring of the fetcher's goroutines coordination so they can be canceled (bt using a context all the way up to the HTTP client) and and the errors can be sent over a channel instead
• Make the logs a bit less verbose in the warning level; only log the first error with an API at this level and log the rest as debug

Related Issue

It fixes some issues reported by one of our customer.

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #802 into master will decrease coverage by 0.54%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #802      +/-   ##
==========================================
- Coverage   21.47%   20.93%   -0.55%     
==========================================
  Files          28       28              
  Lines        2468     2532      +64     
==========================================
  Hits          530      530              
- Misses       1873     1937      +64     
  Partials       65       65

Impacted Files	Coverage Δ
uchiwa/server.go	0% <0%> (ø)	⬆️
uchiwa/daemon/helpers.go	22.22% <0%> (ø)	⬆️
uchiwa/daemon/daemon.go	6.29% <0%> (-1.73%)	⬇️
uchiwa/main.go	43.63% <0%> (ø)	⬆️

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