use own total difficulty to limit best peer

- update blockpool td by subscribing to ChainHeadEvent
- if ahead of best peer, demote it
- addPeer now take own td as current td
- removePeer now take own td as current td
- add relevant tests to peers_test
- eth: backend now calls blockpool with eth.eventMux and chainManager.Td

blockpool/blockpool_util_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/errs"
+	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/pow"
 )
 
@@ -38,6 +39,7 @@ type blockPoolTester struct {
 	blockChain        blockChain
 	blockPool         *BlockPool
 	t                 *testing.T
+	chainEvents       *event.TypeMux
 }
 
 func newTestBlockPool(t *testing.T) (hashPool *test.TestHashPool, blockPool *BlockPool, b *blockPoolTester) {
@@ -48,8 +50,9 @@ func newTestBlockPool(t *testing.T) (hashPool *test.TestHashPool, blockPool *Blo
 		blockChain:        make(blockChain),
 		refBlockChain:     make(blockChain),
 		blocksRequestsMap: make(map[int]bool),
+		chainEvents:       &event.TypeMux{},
 	}
-	b.blockPool = New(b.hasBlock, b.insertChain, b.verifyPoW)
+	b.blockPool = New(b.hasBlock, b.insertChain, b.verifyPoW, b.chainEvents, common.Big0)
 	blockPool = b.blockPool
 	blockPool.Config.BlockHashesRequestInterval = testBlockHashesRequestInterval
 	blockPool.Config.BlocksRequestInterval = testBlocksRequestInterval