Adds coordinator_exception_container which is a typedef over
exception_container and is meant to hold exceptions returned from the
coordinator code path. Currently, it can only hold mutation write
timeout exceptions, because only that kind of error will be returned by
value as a result of this PR. In the future, more exception types can be
added.
Adds coordinator_result which is a boost::outcome::result that uses
coordinator_exception_container as the error type.
9304791ce5