seata 1.4 不能使用Feign的断路器，使用会造成Feign无法传递xid

环境:
seata 1.4
spring-cloud-starter-alibaba-seata 2.0.0

如果Feign使用断路器，断路器会开启一个新的线程。由于xid存在threadLocalMap中，使用断路器会导致threadLocalMap不一致，断路器所在线程的threadLocalMap是空的，这时会取到一个null值，但是feign不开断路器又不行，麻烦项目组看一下这个BUG

Start by reproducing the reported environment with Seata 1.4, spring-cloud-starter-alibaba-seata 2.0.0, Feign, and its circuit breaker enabled. Trace how the xid is stored and accessed across the circuit breaker’s new thread; done means the Feign call still receives the xid while the circuit breaker is enabled.

Written by the indexing model from the issue text.