Fix crashes during CASE establishment.

[bzbarsky-apple]

There were two potential sources of crashes:

1. TryNextResult on the resolver scheduled an async task that could not be canceled. If, while that task was pending, the OperationalSessionSetup was destroyed (which could happen if another session for the same peer had become active in the meantime and another connection attempt happened) we would end up with use-after-free.

2. ScheduleSessionSetupReattempt scheduled a timer that was never being canceled. Similar to above, if the OperationalSessionSetup got destroyed before the timer fired we could end up with use-after-free.

The fix for the first problem is to make TryNextResult synchronous, instead of scheduling an async task.

The fix for the second problem is to introduce a new state of OperationalSessionSetup that's used while waiting for the timer and to ensure that the timer is canceled when leaving that state or if the OperationalSessionSetup is destroyed.

To preserve existing behavior, if we are in the "waiting for timer" state and new connection attempt happens and there is a session we can attach to, then we immediately move to the Connected state (and cancel the timer).

[github-actions]

PR #25868: Size comparison from 745da97 to 0bacd9f

Increases (1 build for cc32xx)

platform	target	config	section	745da97	0bacd9f	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	(read only)	645745	645769	24	0.0
			.debug_info	20309906	20310044	138	0.0
			.debug_line	2680953	2681022	69	0.0
			.debug_str	3041114	3041144	30	0.0
			.symtab	257456	257472	16	0.0
			.text	537672	537696	24	0.0

Decreases (1 build for cc32xx)

platform	target	config	section	745da97	0bacd9f	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_frame	301604	301596	-8	-0.0
			.debug_loc	2827596	2827360	-236	-0.0
			.debug_ranges	286376	286352	-24	-0.0
			.strtab	380555	380543	-12	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	745da97	0bacd9f	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645745	645769	24	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933102	933102	0	0.0
			.debug_aranges	87704	87704	0	0.0
			.debug_frame	301604	301596	-8	-0.0
			.debug_info	20309906	20310044	138	0.0
			.debug_line	2680953	2681022	69	0.0
			.debug_loc	2827596	2827360	-236	-0.0
			.debug_ranges	286376	286352	-24	-0.0
			.debug_str	3041114	3041144	30	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105953	105953	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	380555	380543	-12	-0.0
			.symtab	257456	257472	16	0.0
			.text	537672	537696	24	0.0