We switch to using iterators introduced in Go 1.23
to make iteration alloc-free and fast. And since
element removal is allowed while iterating a map,
this also means we don't need to even copy the slice
any more.
While here, we also address the comment https://github.com/mattermost/mattermost/pull/30178#discussion_r1954862151.
I have simply gone back to using []string as the map
entry rather than a type alias or a redirection with
a struct.
https://mattermost.atlassian.net/browse/MM-63130
```release-note
NONE
```
* Changed back nil to len
```release-note
NONE
```
* fixing unused assignment
```release-note
NONE
```
* add benchmark
```
goos: linux
goarch: amd64
pkg: github.com/mattermost/mattermost/server/v8/channels/app/platform
cpu: Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz
│ old.txt │ new.txt │
│ sec/op │ sec/op vs base │
HubConnIndexIterator/2_users-8 93.53n ± 1% 38.09n ± 1% -59.27% (p=0.000 n=10)
HubConnIndexIterator/3_users-8 106.30n ± 0% 38.41n ± 1% -63.86% (p=0.000 n=10)
HubConnIndexIterator/4_users-8 111.30n ± 1% 38.66n ± 1% -65.27% (p=0.000 n=10)
geomean 103.4n 38.39n -62.89%
│ old.txt │ new.txt │
│ B/op │ B/op vs base │
HubConnIndexIterator/2_users-8 16.00 ± 0% 24.00 ± 0% +50.00% (p=0.000 n=10)
HubConnIndexIterator/3_users-8 24.00 ± 0% 24.00 ± 0% ~ (p=1.000 n=10) ¹
HubConnIndexIterator/4_users-8 32.00 ± 0% 24.00 ± 0% -25.00% (p=0.000 n=10)
geomean 23.08 24.00 +4.00%
¹ all samples are equal
│ old.txt │ new.txt │
│ allocs/op │ allocs/op vs base │
HubConnIndexIterator/2_users-8 1.000 ± 0% 1.000 ± 0% ~ (p=1.000 n=10) ¹
HubConnIndexIterator/3_users-8 1.000 ± 0% 1.000 ± 0% ~ (p=1.000 n=10) ¹
HubConnIndexIterator/4_users-8 1.000 ± 0% 1.000 ± 0% ~ (p=1.000 n=10) ¹
geomean 1.000 1.000 +0.00%
¹ all samples are equal
```
```release-note
NONE
```
* ForChannel test as well
```release-note
NONE
```
* review comments
```release-note
NONE
```
* fix lint errors
```release-note
NONE
```
---------
Co-authored-by: Mattermost Build <build@mattermost.com>
509b8e9af7