core, eth: split eth package, implement snap protocol (#21482)

This commit splits the eth package, separating the handling of eth and snap protocols. It also includes the capability to run snap sync (https://github.com/ethereum/devp2p/blob/master/caps/snap.md

) , but does not enable it by default.

Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>
Co-authored-by: Martin Holst Swende <martin@swende.se>
# Conflicts:
#	cmd/geth/misccmd.go
#	cmd/utils/flags.go
#	core/blockchain.go
#	core/blockchain_snapshot_test.go
#	core/forkid/forkid.go
#	core/rawdb/accessors_snapshot.go
#	core/rawdb/schema.go
#	core/state/snapshot/generate.go
#	core/state/statedb.go
#	eth/api_test.go
#	eth/backend.go
#	eth/downloader/downloader.go
#	eth/downloader/downloader_test.go
#	eth/downloader/modes.go
#	eth/downloader/queue.go
#	eth/downloader/statesync.go
#	eth/gen_config.go
#	eth/handler.go
#	eth/handler_test.go
#	eth/helper_test.go
#	eth/peer.go
#	eth/protocol.go
#	eth/protocol_test.go
#	eth/sync.go
#	eth/sync_test.go
#	ethstats/ethstats.go
#	les/client.go
#	les/enr_entry.go
#	les/handler_test.go
#	les/peer.go
#	les/server_handler.go
#	tests/block_test_util.go
#	trie/proof.go
#	trie/proof_test.go
#	trie/trie.go
#	turbo/trie/notary.go
#	turbo/trie/sync_bloom.go