etcd: STM to allow prefetching of keys and full ranges

This commit adds more full range caching to the STM such that we'll
be able to prefetch keys and ranges on the kvdb level. This will allow
us to reduce roundtrips to etcd do fast iteration of small ranges.

kvdb/etcd/db.go

@@ -219,7 +219,8 @@ func (db *db) View(f func(tx walletdb.ReadTx) error, reset func()) error {
 		return f(newReadWriteTx(stm, etcdDefaultRootBucketId, nil))
 	}
 
-	return RunSTM(db.cli, apply, db.txQueue, db.getSTMOptions()...)
+	_, err := RunSTM(db.cli, apply, db.txQueue, db.getSTMOptions()...)
+	return err
 }
 
 // Update opens a database read/write transaction and executes the function f
@@ -240,7 +241,8 @@ func (db *db) Update(f func(tx walletdb.ReadWriteTx) error, reset func()) error
 		return f(newReadWriteTx(stm, etcdDefaultRootBucketId, nil))
 	}
 
-	return RunSTM(db.cli, apply, db.txQueue, db.getSTMOptions()...)
+	_, err := RunSTM(db.cli, apply, db.txQueue, db.getSTMOptions()...)
+	return err
 }
 
 // PrintStats returns all collected stats pretty printed into a string.