eth: rework tx fetcher to use O(1) ops + manage network requests

core/tx_pool.go

eth/fetcher/block_fetcher.go

eth/fetcher/metrics.go

eth/fetcher/tx_fetcher.go

eth/fetcher/tx_fetcher_test.go

eth/handler.go

eth/metrics.go

eth/peer.go

eth/protocol.go

eth/protocol_test.go

eth/sync.go

fuzzbuzz.yaml

...fetcher/corpus/0151ee1d0db4c74d3bcdfa4f7396a4c8538748c9-2

...fetcher/corpus/020dd7b492a6eb34ff0b7d8ee46189422c37e4a7-6

...fetcher/corpus/021d1144e359233c496e22c3250609b11b213e9f-4

...fetcher/corpus/0d28327b1fb52c1ba02a6eb96675c31633921bb2-2

...fetcher/corpus/0fcd827b57ded58e91f7ba2ac2b7ea4d25ebedca-7

...fetcher/corpus/109bc9b8fd4fef63493e104c703c79bc4a5e8d34-6

...fetcher/corpus/163785ab002746452619f31e8dfcb4549e6f8b6e-6

...fetcher/corpus/1adfa6b9ddf5766220c8ff7ede2926ca241bb947-3