cache result of \OCP\Util::needUpgrade()

reduce calls of \OCP\Util::needUpgrade()
where \OCP\Util::needUpgrade() is called we can call as well
self::checkUpgrade and use the cached result
In line 877 the call way unnecessary anyway because of the first part of
the if statement

move caching to \OCP\Util::needUpgrade

renaming variable

fixing testNeedUpgradeCore()

cache result of checkUpgrade() in self::$needUpgrade

reduce calls of \OCP\Util::needUpgrade()
where \OCP\Util::needUpgrade() is called we can call as well
self::checkUpgrade and use the cached result
In line 877 the call way unnecessary anyway because of the first part of
the if statement

move caching to \OCP\Util::needUpgrade

renaming variable

fixing testNeedUpgradeCore()

fix typo in variable name

deleting tabs