Avoid taking write lock on VortexSession::get

[gatesn]

I was seeing increasing contention from #5662

The alternate and more invasive fix is in #5685 , but we should decide if we want to allowing registering plugins after "startup", and what that means for language bindings that use a global static session object.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.42%. Comparing base (f8870ae) to head (7303f32).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-session/src/lib.rs	87.50%	1 Missing ⚠️

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