lib/db: Configurable block GC time (#6295)

Also retain the interval over restarts by storing last GC time in the
database. This to make sure that GC eventually happens even if the
interval is configured to a long time (say, a month).

cmd/syncthing/main.go

@@ -119,6 +119,10 @@ are mostly useful for developers. Use with care.
                    "h", "m" and "s" abbreviations for hours minutes and seconds.
                    Valid values are like "720h", "30s", etc.
 
+ STGCBLOCKSEVERY   Set to a time interval to override the default database
+                   block GC interval of 13 hours. Same format as the
+                   STRECHECKDBEVERY variable.
+
  GOMAXPROCS        Set the maximum number of CPU cores to use. Defaults to all
                    available CPU cores.