Investigate why `VortexSession` change introduced vortex:duckdb perf regression for clickbench

[0ax1]

https://github.com/vortex-data/vortex/commit/879a53baab7b2a4f72e928fba3cbf0fb92eb989a caused some Clickbench queries to regress for vortex:duckdb.

[0ax1]

@gatesn Do you have a hunch?

[robert3005]

Seems like #5198 was majority of it

[robert3005]

There's still 10% regression in some queries

[gatesn]

So both session and Registry now hold dashmaps. I wonder if that's wrong, and we should actually hold HashMap? Or perhaps the deserialization logic should take an immutable snapshot of the registry prior to deserializing?

[robert3005]

When I was looking at this before the locking cost was non negligible and ideally we’d take immutable snapshot and have a read only version