core/state: access trie through Database interface, track errors (#14589)

With this commit, core/state's access to the underlying key/value database is
mediated through an interface. Database errors are tracked in StateDB and
returned by CommitTo or the new Error method.

Motivation for this change: We can remove the light client's duplicated copy of
core/state. The light client now supports node iteration, so tracing and storage
enumeration can work with the light client (not implemented in this commit).

miner/worker.go

@@ -274,7 +274,7 @@ func (self *worker) wait() {
 				}
 				go self.mux.Post(core.NewMinedBlockEvent{Block: block})
 			} else {
-				work.state.Commit(self.config.IsEIP158(block.Number()))
+				work.state.CommitTo(self.chainDb, self.config.IsEIP158(block.Number()))
 				stat, err := self.chain.WriteBlock(block)
 				if err != nil {
 					log.Error("Failed writing block to chain", "err", err)