brpcServer 异步处理

用echo.proto中的Echo举例，我的需求是，在Echo()运行完后，server能够直接运行一个业务函数，求各位大佬多多指教

Start with the Echo definition in echo.proto and the Echo() handling path described in the issue. Determine how brpc currently schedules work after an RPC handler returns and what API is intended for a follow-up business function. Done should be a documented, agreed behavior with an example and tests, but the issue does not identify relevant source files or tests.

Written by the indexing model from the issue text.