Remove unnecessary &mut self in SQS implementation

[svix-gabriel]

While attempting to use the SQS implementation, I ran into papercuts with the consumer because receive() methods took self by mutable reference.

These, uhh, don't need to do that. I realize the general Consumer traits takes a mutable reference because some underlying implementations require an immutable ref. But when calling the SQS Consumer methods directly, rather than through the consumer trait, this mutable reference is just annoying.

When I made this change, my linter started complaining about unbounded recursion from the macro definitions for implementing the consumer traits. Changing the macros to use the fully qualified struct syntax (e.g. Struct::method instead of &self.method) seems to fix this?