Clean-up symettry in SessionManager

[tcarmelveilleux]

Problem

• SessionManager has symetrical processing steps for
  group and unsecured message, but some side-effects of
  unicast secure sessions are delegated to SecureMessageCodec
  that should actually just be encrypt/decrypt logic.
• This is needed to assist in having a single point of tracing
  for incoming/outgoing messages (upcoming PR). Current organization
  requires putting that logic in several modules due to the
  side-effects occuring at the wrong layer.

Change overview

Done by this PR:

• Hoists those side effects (e.g. updating counter) up from
  SecureMessageCodec where they do not belong, into SessionManager.
• Fix documentation and argument names of SecureMessageCodec that
  had rotted over many refactors and terminological changes.
• Add a bit of processing symmetry in SessionManager

Changes are structural only, and non-functional. Should yield no change
in operation.

Testing

Testing done: unit tests pass, integration tests pass

[github-actions]

PR #13591: Size comparison from 3b8a3c7 to 76cef5c

Increases (2 builds for linux)

platform	target	config	section	3b8a3c7	76cef5c	change	% change
linux	chip-tool-ipv6only	arm64	(read only)	8038388	8038420	32	0.0
			.text	6830532	6830564	32	0.0
	thermostat-no-ble	arm64	(read only)	2041932	2041964	32	0.0
			.text	1697456	1697488	32	0.0

Decreases (1 build for telink)

platform	target	config	section	3b8a3c7	76cef5c	change	% change
telink	lighting-app	tlsr9518adk80d	(read/write)	838430	838406	-24	-0.0
			text	585584	585562	-22	-0.0

Full report (14 builds for efr32, k32w, linux, p6, qpg, telink)

platform	target	config	section	3b8a3c7	76cef5c	change	% change
efr32	lighting-app	BRD4161A	(read only)	832732	832732	0	0.0
			(read/write)	127628	127628	0	0.0
			.bss	125744	125744	0	0.0
			.data	1884	1884	0	0.0
			.text	832724	832724	0	0.0
		BRD4161A+rpc	(read only)	820152	820152	0	0.0
			(read/write)	144288	144288	0	0.0
			.bss	142304	142304	0	0.0
			.data	1984	1984	0	0.0
			.text	820144	820144	0	0.0
	window-app	BRD4161A	(read only)	805392	805392	0	0.0
			(read/write)	126320	126320	0	0.0
			.bss	124480	124480	0	0.0
			.data	1836	1836	0	0.0
			.text	805384	805384	0	0.0
k32w	light	k32w061+release	(read/write)	657888	657888	0	0.0
			.bss	77136	77136	0	0.0
			.data	1852	1852	0	0.0
			.text	573100	573100	0	0.0
	lock	k32w061+release	(read/write)	661572	661572	0	0.0
			.bss	77432	77432	0	0.0
			.data	1872	1872	0	0.0
			.text	576468	576468	0	0.0
linux	chip-tool-ipv6only	arm64	(read only)	8038388	8038420	32	0.0
			(read/write)	370497	370497	0	0.0
			.bss	55217	55217	0	0.0
			.data	1096	1096	0	0.0
			.data.rel.ro	245760	245760	0	0.0
			.dynamic	560	560	0	0.0
			.got	64728	64728	0	0.0
			.init	24	24	0	0.0
			.init_array	168	168	0	0.0
			.rodata	419196	419196	0	0.0
			.text	6830532	6830564	32	0.0
	thermostat-no-ble	arm64	(read only)	2041932	2041964	32	0.0
			(read/write)	145969	145969	0	0.0
			.bss	65089	65089	0	0.0
			.data	880	880	0	0.0
			.data.rel.ro	73016	73016	0	0.0
			.dynamic	560	560	0	0.0
			.got	4048	4048	0	0.0
			.init	24	24	0	0.0
			.init_array	304	304	0	0.0
			.rodata	129884	129884	0	0.0
			.text	1697456	1697488	32	0.0
p6	all-clusters-app	default	(read/write)	2402288	2402288	0	0.0
			.bss	117484	117484	0	0.0
			.data	2592	2592	0	0.0
			.text	1360552	1360552	0	0.0
	light-app	default	(read/write)	2327512	2327512	0	0.0
			.bss	106064	106064	0	0.0
			.data	2392	2392	0	0.0
			.text	1285776	1285776	0	0.0
	lock-app	default	(read/write)	2298424	2298424	0	0.0
			.bss	104920	104920	0	0.0
			.data	2344	2344	0	0.0
			.text	1256688	1256688	0	0.0
qpg	lighting-app	qpg6105+debug	(read only)	561944	561944	0	0.0
			(read/write)	146936	146936	0	0.0
			.bss	89952	89952	0	0.0
			.data	1044	1044	0	0.0
			.text	556624	556624	0	0.0
	lock-app	qpg6105+debug	(read only)	515260	515260	0	0.0
			(read/write)	146936	146936	0	0.0
			.bss	88584	88584	0	0.0
			.data	972	972	0	0.0
			.text	509940	509940	0	0.0
	persistent-storage-app	qpg6105+debug	(read only)	106848	106848	0	0.0
			(read/write)	146940	146940	0	0.0
			.bss	38512	38512	0	0.0
			.data	288	288	0	0.0
			.text	101528	101528	0	0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	838430	838406	-24	-0.0
			bss	87312	87312	0	0.0
			noinit	37160	37160	0	0.0
			text	585584	585562	-22	-0.0