Limit number of items we hold in memory

[svyatonik]

Relayer sometimes eats too much memory (over 512Mb). It happens e.g. when the RPC node connection is slow and it scans large range of headers - then while reading headers, it doesn't read justifications from the subscription and they pile up there. We should check all our queues and either process them in some other way, or have a strict limit

[svyatonik]

could be the reason: paritytech/polkadot-sdk#4481