Use asgiref when available instead of thread locals

[ThePumpingLemma]

Problem Statement
Now that Django is gradually adding async support, thread locals should be eschewed in favor of locals that work in async context.

Describe the solution you'd like
We should use asgiref.local when it is available.

Describe alternatives you've considered
We could use contextvars directly, but this approach is simpler since asgiref is a dependency of newer Django versions.

Additional context
N/A