serialization lengths should be u32

[richard-ramos]

Currently in the code we have places where we do this conversion:

usize::try_from( some_u64_value ).unwrap();

// The u64 value might come from a function like u64::from_be_bytes

This will cause an error in 32 bit architectures, since usize max value depends on the architecture. To avoid this, lengths should be stored as u32 instead of u64

[s1fr0]

This issue was partially discussed in https://github.com/status-im/nwaku/pull/1016#discussion_r905560688. Most of byte lengths converted to usize can be safely assumed to be less than 2^32 bytes, hence we might update RFCs and implementations to deal with length serialized with 4 bytes rather than using 8 bytes were the MSB are set to 0.