refactor: move `handle_future_message()` to `ConversationGuard` [WPB-… #1968

Job	Run time
web benchmarks with bencher	2m 39s
rust benchmarks with bencher (commit)	12m 3s
rust benchmarks with bencher (encryption)	7m 17s
rust benchmarks with bencher (key_package)	5m 6s
rust benchmarks with bencher (create_group)	6m 39s
rust benchmarks with bencher (mls_proteus)	6m 38s
rust benchmarks with bencher (transaction)	5m 28s
	45m 50s

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