fix: block_event dedupe using linked list

[rebelArtists]

Description

• store linked list of seen block hashes and corresponding timestamp withing p2p conn struct
• helper funcs to add new hash to linked list and lookup seen hashes
• automatic TTL deletion of stale hashes

Testing

smoke tests locally (i.e. no go errors)

prod tests w/ datastore also appear successful:
hash=0x49858d9863bd02bebec5805f0902e73342fa0846ca05e9c641292af00bc94a8f peer=enode://360811f7d432cc06f28b54ca09d57a374b94648d8eed6a8364190ec8e0bdbb43402cafda9f579a50a0fbc38448672a81ba1eaf5965728688ed0731993e920ae9@200.69.14.233:32303 2:43PM INF Skipping duplicate block hash hash=0x49858d9863bd02bebec5805f0902e73342fa0846ca05e9c641292af00bc94a8f peer=enode://360811f7d432cc06f28b54ca09d57a374b94648d8eed6a8364190ec8e0bdbb43402cafda9f579a50a0fbc38448672a81ba1eaf5965728688ed0731993e920ae9@200.69.14.233:32303 2:43PM INF Skipping duplicate block hash hash=0x49858d9863bd02bebec5805f0902e73342fa0846ca05e9c641292af00bc94a8f peer=enode://360811f7d432cc06f28b54ca09d57a374b94648d8eed6a8364190ec8e0bdbb43402cafda9f579a50a0fbc38448672a81ba1eaf5965728688ed0731993e920ae9@200.69.14.233:32303

datastore shows impact of deduped msgs by testing count of Key after change:

also mem has increased <5% locally after turning on sensor and letting run >10min:

if we further refine datastore query, we see only a single record for each unique Hash + PeerId combo:

[minhd-vu]

Awesome work Dan. Note that, this will reduce block events, but there is an edge case where a peer can send a NewBlock and BlockHashes message. A NewBlock message will also write a block event to Datastore. This shouldn't be a concern because a well behaving peer will never propagate the block hash to our sensor if it's already sent the full block. This is just something to be aware of. See the image below, since we recieve so few NewBlock messages, it should also not be a concern in terms of duplication.