Fix issues #2810, #2813, #2822, #2829 (#2816)

* Add isValidUTCDateTime function to validate timestamp as received from OneDrive API to ensure it is valid
* Use new function before attempting to call SysTime.fromISOExtString to ensure this call will be successful
* If there is no timestamp in the JSON, set it to the system time
* Add assertion when building an item from DB data
* Add new function (isValidUTF8) to check UTF-8 validity of a string before timestamp regex check
* In a --resync scenario, if the file hash is the same, use the online timestamp as source of truth
* Ensure that the session URL data is a valid JSON response before use
* Ensure a local time in UTC is being used if the JSON data has no date
* Ensure the DB is opened in the most threadsafe manner possible
* Add patch provided by @phlibi to add synchronized() around DB access methods
* Align timestamp creation method with itemdb if element is missing

Author: abraunegg