Cache: Fix a whole lot of things.

* Minor updates to the min & max cache sizes (16 & 64MB). Mostly to satisfy my power-of-two OCD.
  * Purge broken on-disk cache files
  * Optimize free RAM computations
  * Start dropping LRU items when running low on memory before pre-rendring (hinting) pages in non-reflowable documents.
  * Make serialize dump the most recently *displayed* page, as the actual MRU item is the most recently *hinted* page, not the current one.
  * Use more accurate item size estimations across the whole codebase.

TileCacheItem:

  * Drop lua-serialize in favor of Persist.

KoptInterface:

  * Drop lua-serialize in favor of Persist.
  * Make KOPTContext caching actually work by ensuring its hash is stable.

frontend/apps/reader/readerui.lua

@@ -733,13 +733,13 @@ function ReaderUI:onClose(full_refresh)
     if self.dialog ~= self then
         self:saveSettings()
     end
+    -- serialize last used items for later launch
+    Cache:serialize()
     if self.document ~= nil then
         logger.dbg("closing document")
         self:notifyCloseDocument()
     end
     UIManager:close(self.dialog, full_refresh and "full")
-    -- serialize last used items for later launch
-    Cache:serialize()
     if _running_instance == self then
         _running_instance = nil
     end