Implement database abstraction, error checking (ref #5907) (#6107)

This PR does two things, because one lead to the other:

- Move the leveldb specific stuff into a small "backend" package that
defines a backend interface and the leveldb implementation. This allows,
potentially, in the future, switching the db implementation so another
KV store should we wish to do so.

- Add proper error handling all along the way. The db and backend
packages are now errcheck clean. However, I drew the line at modifying
the FileSet API in order to keep this manageable and not continue
refactoring all of the rest of Syncthing. As such, the FileSet methods
still panic on database errors, except for the "database is closed"
error which is instead handled by silently returning as quickly as
possible, with the assumption that we're anyway "on the way out".

lib/db/meta.go

@@ -57,7 +57,10 @@ func (m *metadataTracker) Marshal() ([]byte, error) {
 // toDB saves the marshalled metadataTracker to the given db, under the key
 // corresponding to the given folder
 func (m *metadataTracker) toDB(db *instance, folder []byte) error {
-	key := db.keyer.GenerateFolderMetaKey(nil, folder)
+	key, err := db.keyer.GenerateFolderMetaKey(nil, folder)
+	if err != nil {
+		return err
+	}
 
 	m.mut.RLock()
 	defer m.mut.RUnlock()
@@ -70,7 +73,7 @@ func (m *metadataTracker) toDB(db *instance, folder []byte) error {
 	if err != nil {
 		return err
 	}
-	err = db.Put(key, bs, nil)
+	err = db.Put(key, bs)
 	if err == nil {
 		m.dirty = false
 	}
@@ -81,8 +84,11 @@ func (m *metadataTracker) toDB(db *instance, folder []byte) error {
 // fromDB initializes the metadataTracker from the marshalled data found in
 // the database under the key corresponding to the given folder
 func (m *metadataTracker) fromDB(db *instance, folder []byte) error {
-	key := db.keyer.GenerateFolderMetaKey(nil, folder)
-	bs, err := db.Get(key, nil)
+	key, err := db.keyer.GenerateFolderMetaKey(nil, folder)
+	if err != nil {
+		return err
+	}
+	bs, err := db.Get(key)
 	if err != nil {
 		return err
 	}