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/ui/message/streammessagequeue.lua

@@ -57,11 +57,11 @@ end
 
 function StreamMessageQueue:waitEvent()
     local data = ""
-    -- Successive zframes may be tens or hundreds in some cases
-    -- if they are concatenated in a single loop it may run up memory of the
-    -- machine. And it did happened when receiving file data from Calibre server.
-    -- Here we receive only receive 10 packages at most in one waitEvent loop, and
-    -- call receiveCallback immediately.
+    -- Successive zframes may come in batches of tens or hundreds in some cases.
+    -- If they are concatenated in a single loop, it may consume a significant amount
+    -- of memory. And it's fairly easy to trigger when receiving file data from Calibre.
+    -- So, throttle reception to 10 packages at most in one waitEvent loop,
+    -- after which we immediately call receiveCallback.
     local wait_packages = 10
     while czmq.zpoller_wait(self.poller, 0) ~= nil and wait_packages > 0 do
         local id_frame = czmq.zframe_recv(self.socket)