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/api_backend.go

@@ -174,8 +174,15 @@ func (b *LesApiBackend) AccountManager() *accounts.Manager {
 }
 
 func (b *LesApiBackend) BloomStatus() (uint64, uint64) {
-	return params.BloomBitsBlocks, 0
+	if b.eth.bloomIndexer == nil {
+		return 0, 0
+	}
+	sections, _, _ := b.eth.bloomIndexer.Sections()
+	return light.BloomTrieFrequency, sections
 }
 
 func (b *LesApiBackend) ServiceFilter(ctx context.Context, session *bloombits.MatcherSession) {
+	for i := 0; i < bloomFilterThreads; i++ {
+		go session.Multiplex(bloomRetrievalBatch, bloomRetrievalWait, b.eth.bloomRequests)
+	}
 }