statement-store: Processing statements received from network is inefficient on a big number of statements

[AndreiEres]

TODO: This will be inneficient if number of statement_subscriptions is big. I assume it shouldn't.

Originally posted by @alexggh in #3127 (comment)

smoldot/light-base/src/json_rpc_service/background.rs

Lines 753 to 754 in 6b792e8

	// TODO: not efficient on a big number of subscriptions
	for (sub_id, topic_filter) in &me.statement_subscriptions {