Consolidate locking mechanisms with stdlib Mutex

[finestructure]

We're using three different locking mechanisms at the moment, ActorIsolated, LockIsolated, and QueueIsolated. We should use the new standard library Mutex instead.

[finestructure]

Have to be aware of some pitfalls here, Mutex can apparently crash the compiler on Linux.

https://github.com/swift-server/swift-aws-lambda-runtime/blob/eb634fa9abcd178f9f86852ea0839109bd949a97/Sources/AWSLambdaRuntime/LambdaRuntime.swift#L30

[finestructure]

See swiftlang/swift#80036 and swiftlang/swift#80020

[sebsto]

Looks like the compiler crash happens on Mutex with generic types only
swiftlang/swift#78048

[sebsto]

I also think swiftlang/swift#80020 is not related to Mutex