Mark CallExecutor as ThreadUnsafe or force user to create CallExecutor in every execute call to avoid data race

[jeremyfu-aws]

Since CallExecutor has status as instance variable to store result, it is needed to create new CallExecutor in every execute call, otherwise it could cause data race under multi-threaded environment.

In order to avoid the misuse of CallExecutor, please consider to

• add ThreadUnsafe annotation on CallExecutor
• or implement in a way which forces users to create new CallExecutor in every execute call

[elennick]

@jeremyfu-aws i am no longer going to be maintaining retry4j - however please feel free to fork and make whatever changes you'd like