Unify ReadFieldNew / ReadFieldUpdate

[ryanofsky]

Replace with ReadField wrapper and CustomReadField hook to be consistent with BuildField / CustomBuildField. This

• Makes it more straightforward pass custom types with symmetric CustomReadField / CustomBuildField overloads

• Simplifies debugging and overload resolution, removing hacks that allowed ReadFieldNew calls to forward to ReadFieldUpdate calls and vice versa

• Enables more flexibility and efficiency within CustomReadField implementations allowing emplacing or updating in the same function based on which is simpler or more efficient. Removes a lot of std::enable_if overload hacks

• Should allow clientInvoke to use RVO and construct return values in place instead of copying or moving. This will allow more types to be for supported as IPC return values even if they don't support default construction, copying, or moving.

• Improves separation of concerns, avoids need for ReadField callers to know whether callee will emplace or update, avoids need for emplace ReadDest object to have hardcoded support for map, vector, optional, shared_ptr, reference_wrapper types, so they now are implemented the same as any other custom type.

[ryanofsky]

Corresponding updates for bitcoin were https://github.com/ryanofsky/bitcoin/commit/cc91cff97548f24a169eea0307590be8d967c5c8