[fleet_executor] Add sync method

[FeixLiu]

PR types

Others

PR changes

Others

Describe

Add sync method.

If Carrier receives messages during creating interceptors, it may encounter a situation where the target interceptor has not been initialized, then it will fall into a RunTime error (dst interceptor was not found). To avoid this happen, we add a new flag creating_interceptors which indicating all interceptors all still under initialization.

For now, if Carrier receives messages during creating interceptors, it will first push those messages into a tmp vector. After creating all interceptors, the Carrier will handle all messages in the vector.

Three usages:

No. 1

User provides an init config, and won't set new interceptors later.
No extra codes are needed.

No. 2

User doesn't provide an init config, and will set interceptors later by calling SetInterceptor.
After adding all interceptors, method SetCreatingFlag(false) needs to be called.

No. 3

User provides an init config, and will set more interceptors later by calling SetInterceptor.
Before adding new interceptors, method SetCreatingFlag(true) needs to be called.
After adding all interceptors, method SetCreatingFlag(false) needs to be called.

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[wangxicoding]

LGTM