Start Merry-Go-Round write-up and fix visualisations (#463)

* Initial commit

* Bit more

* Fix illustrations

* More on MDG

docs/merry-go-round-sync.md

@@ -0,0 +1,25 @@
+# Merry-Go-Round sync (turbo-geth design)
+
+Initial discussion here: https://ethresear.ch/t/merry-go-round-sync/7158
+
+## Participants
+
+We divide participants into two types: seeders and leechers. Seeders are the ones who have the full copy of the current Ethereum
+state, and they also keep updating it as new blocks appear. Leechers are the ones that join the Merry-Go-Round network to
+either acquire the full copy of the state, or "sift" through it to acquire some part of the state (this is what some "Stateless"
+node might like to do in the beginning).
+
+## Syncing cycles
+
+The process of syncing happens in cycles. During each cycle, all participants attempt to distribute the entire content of the
+Ethereum state to each other. Therefore the duration of one cycle for the Ethereum mainnet is likely to be some hours.
+Cycle is divided into ticks. For convinience, we can say that each ticks starts after the mining of certain block, and lasts
+for some predetermined amount of time, let's say, 30 seconds. This definition means that the ticks will often overlap, but not
+always, as shown onn the picture below.
+
+![cycles-and-ticks](mgr-sync-1.png)
+
+
+
+## How will seeders produce sync schedule
+

trie/resolver_stateful_cached.go

@@ -333,11 +333,18 @@ func (tr *ResolverStatefulCached) MultiWalk2(db *bolt.DB, blockNr uint64, bucket
 	startkey := startkeys[rangeIdx]
 
 	err := db.View(func(tx *bolt.Tx) error {
-		cache := tx.Bucket(dbutils.IntermediateTrieHashBucket).Cursor()
+		cacheBucket := tx.Bucket(dbutils.IntermediateTrieHashBucket)
+		var cache *bolt.Cursor
+		if cacheBucket != nil {
+			cache = cacheBucket.Cursor()
+		}
 		c := tx.Bucket(bucket).Cursor()
 
 		k, v := c.Seek(startkey)
-		cacheK, cacheV := cache.Seek(startkey)
+		var cacheK, cacheV []byte
+		if cache != nil {
+			cacheK, cacheV = cache.Seek(startkey)
+		}
 
 		var minKey []byte
 		var fromCache bool