Support migrating existing values

Re-rolled against 2.0.x

I wonder how this issue will be affected by 🐛 Serial not generating on entity save in certain contexts Needs work . I am adding that as a related issue.

Looking at the patch in #23, I have a few comments.

1.
+++ b/src/SerialSQLStorage.php
@@ -73,7 +73,7 @@ class SerialSQLStorage implements ContainerInjectionInterface, SerialStorageInte
   /**
    * {@inheritdoc}
    */
-  public function generateValueFromName($storageName, $delete = TRUE) {
+  public function generateValueFromName($storageName, $externalValue = NULL, $delete = TRUE) {

Normally, when adding a new optional parameter, we add it after the existing parameters, for backwards compatibility (BC).

2. @@ -81,9 +81,17 @@ class SerialSQLStorage implements ContainerInjectionInterface, SerialStorageInte
...
+      if (isset($externalValue)) {
+        $sid = $connection->insert($storageName)
+          ->fields(['sid' => $externalValue, 'uniqid' => $uniqid])
+          ->execute();
+      }
+      else {
+        $sid = $connection->insert($storageName)
+          ->fields(['uniqid' => $uniqid])
+          ->execute();
+      }

This can be simplified. Something like this:

$values = [...];
if (...) {
  $values['sid'] = ...;
}
$sid = $connection->insert(...)
  ->fields($values)
  ->execute();