Fix endless loop on shutdown when DB can't be reached

The shutdown handler calls `checkAllNotesRevision` on a 100 ms
interval. If the database connection is broken, this will return
an error. Previously, this error was effectively ignored and resulted
in an endless loop printing out the error message every 100 ms.

This improves the error handling by terminating the process with a
nonzero exit code when an error was encountered 30 times. The loop
interval is also increased to 200 ms, giving the database 6 seconds
total time to recover in case of intermittent issues.

Signed-off-by: David Mehren <git@herrmehren.de>

public/docs/release-notes.md

@@ -11,6 +11,7 @@
 
 ### Bugfixes
 - Fix crash when trying to read the current Git commit on startup 
+- Fix endless loop on shutdown when HedgeDoc can't connect to the database
 
 ## <i class="fa fa-tag"></i> 1.8.2 <i class="fa fa-calendar-o"></i> 2021-05-11