Calibre: Minor QoL fixes (#7528)

* CalibreMetadata: Get rid of the now useless NULL-hunt: here, this was basically looking for `rapidjson.null` to replace them with... `rapidjson.null` :?. IIRC, that's a remnant of a quirk of the previous JSON parser (possibly even the previous, *previous* JSON parser ^^).
* CalibreSearch: Update the actually relevant NULL-hunt to make it explicit: replace JSON NULLs with Lua nils, instead of relying on an implementation detail of Lua-RapidJSON, because that detail just changed data type ;).
* UIManager: Make sure tasks scheduled during the final ZMQ callback are honored. e.g., the Calibre "Disconnect" handler. This happened to mostly work purely by chance before the event loop rework.
* Calibre: Restore a proper receiveCallback handler after receiving a book, in order not to break the "Disconnect" handler's state (and, well, get a working Disconnect handler, period ^^).
* Calibre: Unbreak metadata cache when it's initialized by a search (regression since #7159).
* Calibre: Handle UTC <-> local time conversions when checking the cache's timestamp against the Calibre metadata timestamp.
* Bump base (Unbreak CRe on Android, update RapidJSON)

frontend/cache.lua

@@ -41,6 +41,9 @@ 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