lib/db: Refactor: use a Lowlevel type underneath Instance (ref #5198) (#5212)

This adds a thin type that holds the state associated with the
leveldb.DB, leaving the huge Instance type more or less stateless. Also
moves some keying stuff into the DB package so that other packages need
not know the keying specifics.

(This does not, yet, fix the cmd/stindex program, in order to keep the
diff size down. Hence the keying constants are still exported.)

lib/db/keyer_test.go

@@ -16,7 +16,7 @@ func TestDeviceKey(t *testing.T) {
 	dev := []byte("device67890123456789012345678901")
 	name := []byte("name")
 
-	db := OpenMemory()
+	db := newInstance(OpenMemory())
 
 	key := db.keyer.GenerateDeviceFileKey(nil, fld, dev, name)
 
@@ -44,7 +44,7 @@ func TestGlobalKey(t *testing.T) {
 	fld := []byte("folder6789012345678901234567890123456789012345678901234567890123")
 	name := []byte("name")
 
-	db := OpenMemory()
+	db := newInstance(OpenMemory())
 
 	key := db.keyer.GenerateGlobalVersionKey(nil, fld, name)
 
@@ -69,7 +69,7 @@ func TestGlobalKey(t *testing.T) {
 func TestSequenceKey(t *testing.T) {
 	fld := []byte("folder6789012345678901234567890123456789012345678901234567890123")
 
-	db := OpenMemory()
+	db := newInstance(OpenMemory())
 
 	const seq = 1234567890
 	key := db.keyer.GenerateSequenceKey(nil, fld, seq)