lib/db: Use Commit() instead of commit() (#6330)

The readWriteTransaction offered both commit() (the one to use) and
Commit() (via embedding) where the latter didn't close the read
transaction. This removes the lower cased variant in order to prevent
the mistake.

The only place where the mistake was made was the new gc runner, where
it would leave a read snapshot open forever.

lib/db/blockmap_test.go

@@ -76,7 +76,7 @@ func addToBlockMap(db *Lowlevel, folder []byte, fs []protocol.FileInfo) error {
 			}
 		}
 	}
-	return t.commit()
+	return t.Commit()
 }
 
 func discardFromBlockMap(db *Lowlevel, folder []byte, fs []protocol.FileInfo) error {
@@ -101,7 +101,7 @@ func discardFromBlockMap(db *Lowlevel, folder []byte, fs []protocol.FileInfo) er
 			}
 		}
 	}
-	return t.commit()
+	return t.Commit()
 }
 
 func TestBlockMapAddUpdateWipe(t *testing.T) {