trie: rename TrieSync to Sync and improve hexToKeybytes (#16804)

This removes a golint warning: type name will be used as trie.TrieSync by
other packages, and that stutters; consider calling this Sync.

In hexToKeybytes len(hex) is even and (even+1)/2 == even/2, remove the +1.

eth/downloader/statesync.go

@@ -214,7 +214,7 @@ func (d *Downloader) runStateSync(s *stateSync) *stateSync {
 type stateSync struct {
 	d *Downloader // Downloader instance to access and manage current peerset
 
-	sched  *trie.TrieSync             // State trie sync scheduler defining the tasks
+	sched  *trie.Sync                 // State trie sync scheduler defining the tasks
 	keccak hash.Hash                  // Keccak256 hasher to verify deliveries with
 	tasks  map[common.Hash]*stateTask // Set of tasks currently queued for retrieval