use cl.ID instead of pk.Connect.ClientIdentifier when looking for existing clients in inheritClientSession

[SimonMacIntyre]

Fix https://github.com/mochi-mqtt/server/issues/416

The reason I want to open this PR:

• cl.ID is set from the pk.Connect.ClientIdentifier in clients.go, It also is the same value as the assignedClientId if there is one. Therefore it is more correct, but also for the reasons below:
• This allows my custom implementation to rely on cl.ID and override it and the assigned id returned to the client(I assign intentional, known, client ids to specific clients. We want to control the client ids based on the authenticating client.)

All tests still pass, including the existing client takeover tests.

@mochi-co Do you think the existing tests are good for this, or do we want some kind of regression test case? I was looking at the tests to add one but to be honest, it is a bit beyond my Go skill level at the moment.

[SimonMacIntyre]

Huh, not sure I understand the pipeline failure, the tests all worked on this locally 🤔 Anything I can do?

[coveralls]

Pull Request Test Coverage Report for Build 10167435003

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.623%

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Totals
Change from base Build 10167421293:	0.0%
Covered Lines:	6160
Relevant Lines:	6246

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💛 - Coveralls

[mochi-co]

@SimonMacIntyre Looks like it was just flakey tests 👍🏻

[mochi-co]

Nice improvement here, thank you @SimonMacIntyre :)