Fix potential NextPacketID endless loop, expand tests

[mochi-co]

Fixes potential endless loop scenario with clients.NextPacketID and updates related tests to cover this scenario.
cc @thedevop Thank you for your code sample - it really helped illustrate the issue!

[thedevop]

This works. However, there is a slightly optimized solution:
Change:

	i = atomic.LoadUint32(&cl.State.packetID)
	if i >= 65535 {
		i = 0
	}

	started := i + 1

to

	i = atomic.LoadUint32(&cl.State.packetID)
	started := i

With started := i + 1, started will be checked twice.

Example with started := i
Example with started := i + 1

[mochi-co]

@thedevop I believe this optimization will put the method into an infinite loop if all packet ids are allocated, as started can be 65535 but if overflowed && i == started { can never be reached.

[mochi-co]

Optimization was implemented by refactoring NextPacketID, and small changes were made to enhance ease of testing (introduction of unexported Capabilities.maximumPacketID field). Additionally, max uint16 values were replaced with math constants.