Don't update value if it is already set to the same value

* this PR makes sure to warm up the cache for that user
* then the logic within the "if is in cache" code can be used to reduce needed queries
* inspired by @andreas-p - https://github.com/nextcloud/server/pull/2128

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

lib/private/AllConfig.php

@@ -215,11 +215,14 @@ class AllConfig implements \OCP\IConfig {
 		// TODO - FIXME
 		$this->fixDIInit();
 
+		// warm up the cache to avoid updating the value if it is already set to this value before
+		$this->getUserValue($userId, $appName, $key);
+
 		if (isset($this->userCache[$userId][$appName][$key])) {
 			if ($this->userCache[$userId][$appName][$key] === (string)$value) {
 				return;
 			} else if ($preCondition !== null && $this->userCache[$userId][$appName][$key] !== (string)$preCondition) {
-				return;
+				throw new PreConditionNotMetException();
 			} else {
 				$qb = $this->connection->getQueryBuilder();
 				$qb->update('preferences')