les, light: LES/2 protocol version (#14970)

This PR implements the new LES protocol version extensions:

* new and more efficient Merkle proofs reply format (when replying to
  a multiple Merkle proofs request, we just send a single set of trie
  nodes containing all necessary nodes)
* BBT (BloomBitsTrie) works similarly to the existing CHT and contains
  the bloombits search data to speed up log searches
* GetTxStatusMsg returns the inclusion position or the
  pending/queued/unknown state of a transaction referenced by hash
* an optional signature of new block data (number/hash/td) can be
  included in AnnounceMsg to provide an option for "very light
  clients" (mobile/embedded devices) to skip expensive Ethash check
  and accept multiple signatures of somewhat trusted servers (still a
  lot better than trusting a single server completely and retrieving
  everything through RPC). The new client mode is not implemented in
  this PR, just the protocol extension.

les/retrieve.go

@@ -22,6 +22,7 @@ import (
 	"context"
 	"crypto/rand"
 	"encoding/binary"
+	"fmt"
 	"sync"
 	"time"
 
@@ -111,12 +112,14 @@ func newRetrieveManager(peers *peerSet, dist *requestDistributor, serverPool pee
 // that is delivered through the deliver function and successfully validated by the
 // validator callback. It returns when a valid answer is delivered or the context is
 // cancelled.
-func (rm *retrieveManager) retrieve(ctx context.Context, reqID uint64, req *distReq, val validatorFunc) error {
+func (rm *retrieveManager) retrieve(ctx context.Context, reqID uint64, req *distReq, val validatorFunc, shutdown chan struct{}) error {
 	sentReq := rm.sendReq(reqID, req, val)
 	select {
 	case <-sentReq.stopCh:
 	case <-ctx.Done():
 		sentReq.stop(ctx.Err())
+	case <-shutdown:
+		sentReq.stop(fmt.Errorf("Client is shutting down"))
 	}
 	return sentReq.getError()
 }