Centralize one time migration code after updates (#7531)

There have been a couple of these this month, and keeping stuff that should only ever run once piling up in their respective module was getting ugly, especially when it's usually simple stuff (settings, files).

So, move everything to a dedicated module, run by reader.lua on startup, and that will actually only do things once, when necessary.

frontend/cache.lua

@@ -39,12 +39,6 @@ end
 
 local cache_path = DataStorage:getDataDir() .. "/cache/"
 
--- NOTE: Before 2021.04, fontlist used to squat our folder, needlessly polluting our state tracking.
-os.remove(cache_path .. "/fontinfo.dat")
--- Ditto for Calibre
-os.remove(cache_path .. "/calibre-libraries.lua")
-os.remove(cache_path .. "/calibre-books.dat")
-
 --[[
 -- return a snapshot of disk cached items for subsequent check
 --]]
@@ -160,7 +154,7 @@ function Cache:serialize()
     -- calculate disk cache size
     local cached_size = 0
     local sorted_caches = {}
-    for _,file in pairs(self.cached) do
+    for _, file in pairs(self.cached) do
         table.insert(sorted_caches, {file=file, time=lfs.attributes(file, "access")})
         cached_size = cached_size + (lfs.attributes(file, "size") or 0)
     end
@@ -203,4 +197,9 @@ function Cache:clear()
     self.current_memsize = 0
 end
 
+-- Refresh the disk snapshot (mainly used by ui/data/onetime_migration)
+function Cache:refreshSnapshot()
+    self.cached = getDiskCache()
+end
+
 return Cache