refactor(core): Merge filestream "loop_" files

[timoffex]

Description

Merges loop_process.go, loop_transmit.go and loop_feedback.go into a single filestreamimpl.go (open to naming suggestions). This makes the data flow a bit easier to see. One can think of these three goroutines as a single function that processes Updates, so it helps to see all the processing steps an Update undergoes in one place.

I also made each loop function close its corresponding output stream on its own so that channels are closed near where they're pushed to. As a side effect, this simplifies filestream.Close().

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 75.85%. Comparing base (8c5ee31) to head (c8acb20).

Additional details and impacted files

@@                                       Coverage Diff                                       @@
##           spr/timoffex/main.refactorcore-merge-filestream-loop_-files    #7624      +/-   ##
===============================================================================================
+ Coverage                                                        75.81%   75.85%   +0.03%     
===============================================================================================
  Files                                                              502      500       -2     
  Lines                                                            54048    54026      -22     
===============================================================================================
+ Hits                                                             40979    40981       +2     
+ Misses                                                           12660    12636      -24     
  Partials                                                           409      409              

Flag	Coverage Δ
func	41.18% <ø> (+0.07%)	⬆️
system	63.49% <85.71%> (+0.01%)	⬆️
unit	56.38% <85.71%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
core/pkg/filestream/filestream.go	85.50% <100.00%> (-2.45%)	⬇️
core/pkg/filestream/filestreamimpl.go	86.27% <82.60%> (ø)

... and 8 files with indirect coverage changes