stream/buffer: Improve orderbook verification conditions and fix TestUpdateByIDAndAction test (#1682)

* don't need to verify if checksum available; don't retroactively verify snapshots

* glorious: nits + allow checksum/validation to be done on updateByIDAndAction

* possible slice manipulation issue fix for test

* thrasher: nit

* this should fix it now

* for subsystem usage disallow deploying a book twice

* reverts strict policy using deploy, it's used in a bunch of tests I don't want to touch just yet, left a note

* rm unused variable

* glorious: nits

---------

Co-authored-by: Ryan O'Hara-Reid <ryan.oharareid@thrasher.io>

exchanges/orderbook/orderbook.go

@@ -79,8 +79,7 @@ func (s *Service) Update(b *Base) error {
 	return s.Mux.Publish(book, m1.ID)
 }
 
-// DeployDepth used for subsystem deployment creates a depth item in the struct
-// then returns a ptr to that Depth item
+// DeployDepth used for subsystem deployment creates a depth item in the struct then returns a ptr to that Depth item
 func (s *Service) DeployDepth(exchange string, p currency.Pair, a asset.Item) (*Depth, error) {
 	if exchange == "" {
 		return nil, errExchangeNameUnset
@@ -112,13 +111,15 @@ func (s *Service) DeployDepth(exchange string, p currency.Pair, a asset.Item) (*
 		s.books[strings.ToLower(exchange)] = m1
 	}
 	book, ok := m1.m[mapKey]
-	if !ok {
-		book = NewDepth(m1.ID)
-		book.exchange = exchange
-		book.pair = p
-		book.asset = a
-		m1.m[mapKey] = book
+	if ok {
+		// Maybe in future we should return an error here and be more strict.
+		return book, nil
 	}
+	book = NewDepth(m1.ID)
+	book.exchange = exchange
+	book.pair = p
+	book.asset = a
+	m1.m[mapKey] = book
 	return book, nil
 }