perf: improve locking code

[mattjohnsonpint]

Description

There are a few places where we can improve locking code. Specifically:

• Replace double-checked locking on singleton object instantiation with sync.Once
• Replace double-checked locking on maps with a concurrent map
• Ensure defer is used correctly to release mutex locks where still used

For the concurrent map, I evaluated both sync.Map and xsync.MapOf, and decided on the latter.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to this PR