Fix unsafe threadid indexing

[NHDaly]

See: https://julialang.org/blog/2023/07/PSA-dont-use-threadid/

This is unsafe now that julia has the ability to add and remove threads dynamically; you may end up on a threadid that is out-of-bounds on the vector.

[NHDaly]

@kpamnany suggests that the proper fix is to just shard your cache into several smaller caches.

The goal of having this split cache is just to reduce contention. You can do that with any key that shards the space; it doesn't need to be a threadid. For example, you can hash the taskid or shard by the keyspace itself, etc etc.

(Sharding by keyspace is what our similar cache that Todd wrote in RAICode does.)

[NHDaly]

CC: @ttonelli