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>

trie/notary.go

@@ -0,0 +1,57 @@
+// Copyright 2020 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package trie
+
+import (
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/ethdb/memorydb"
+)
+
+// KeyValueNotary tracks which keys have been accessed through a key-value reader
+// with te scope of verifying if certain proof datasets are maliciously bloated.
+type KeyValueNotary struct {
+	ethdb.KeyValueReader
+	reads map[string]struct{}
+}
+
+// NewKeyValueNotary wraps a key-value database with an access notary to track
+// which items have bene accessed.
+func NewKeyValueNotary(db ethdb.KeyValueReader) *KeyValueNotary {
+	return &KeyValueNotary{
+		KeyValueReader: db,
+		reads:          make(map[string]struct{}),
+	}
+}
+
+// Get retrieves an item from the underlying database, but also tracks it as an
+// accessed slot for bloat checks.
+func (k *KeyValueNotary) Get(key []byte) ([]byte, error) {
+	k.reads[string(key)] = struct{}{}
+	return k.KeyValueReader.Get(key)
+}
+
+// Accessed returns s snapshot of the original key-value store containing only the
+// data accessed through the notary.
+func (k *KeyValueNotary) Accessed() ethdb.KeyValueStore {
+	db := memorydb.New()
+	for keystr := range k.reads {
+		key := []byte(keystr)
+		val, _ := k.KeyValueReader.Get(key)
+		db.Put(key, val)
+	}
+	return db
+}