Fix Protected Value Property Access Safety

[jshier]

Goals ⚽

This PR refactors the codebase to always access Protected properties through the wrapper when accessing sub properties. That is protectedValue.property must always be accessed as $protectedValue.property to the full access is protected by the lock.

Essentially, lock.around { value } is roughly equivalent to

lock.lock()
return value
lock.unlock()

which necessarily puts any access to the returned value's properties outside the lock. Instead, using the KeyPath-based dynamicMemberLookup ensures the entire access is protected, as it's roughly equivalent to:

lock.lock()
return value[keyPath: keyPath]
lock.unlock()

Implementation Details 🚧

In addition to auditing the use sites, various Protected APIs were updated to support throws/rethrows, to enable them to wrap throwing APIs. Namely, MultipartUpload has some troublesome internal API that access a Protected value while building the multipart payload, meaning the entire access must be protected. Both of these (let data = try $multipartFormData.read { try $0.encode() } and try $multipartFormData.read { try $0.writeEncodedData(to: fileURL) }) perform unbounded work inside the lock, which could have troublesome performance implications. However, without extensive refactoring of the multipart payload APIs, changes to the functionality isn't possible. This work was already being performed in the background, so overall impact should be minimal.

Testing Details 🔍

Additional tests were added, including one larger, high contention test which triggered the thread sanitizer when using a wrapped value instead of the wrapper.

[cnoon]

Looks great @jshier, nice work!