Use safe datacache format

[dvz]

Convert the format of default datacache storage from PHP's own serialize format to improve safety and performance.

JSON or similar formats may be used instead.

Related:

• Use PHP's own unserialize() for performance reasons in chosen locations #4702
• user input parsed using my_serialize(): 7244545

[RevertIT]

I'll be blunt here, but Datacache needs to go and be replaced with new and modern caching handlers. It's actually designed very poorly and it's so easy to hit a bottleneck.

1. For example my plugin for disposable emails hits several limitations:
   1. mediumtext storing type and serialization.
   2. Site-wide query SELECT title,cache FROM mybb_datacache, so when there are some heavy caches, the query executes for like 3+ seconds. (Again my plugin which I had to split in chunks, still getting bottlenecked because of it even when it's disabled)
   3. Heavy if/else checks with datacache and respective handlers.
   4. Storing all cache functions inside datacache class doesn't help as well, it should be stored and read on-demand.