Revise dispose pattern in SpanProcessor/ActivityProcessor

[rajkumar-rangaraj]

Dispose pattern in SimpleSpanProcessor, SimpleActivityProcessor, BatchingSpanProcessor and BatchingActivityProcessor needs to be revised. As @reyang mentioned ShutdownAsync is not thread safe, race condition could happen for this.stopping, and it would re-enter to re-execute the same code.

Raised as a part of PR: #755 (comment)

[eddynaka]

@reyang , for this, do you think a simple lock in a variable in the dispose method would work? Or do you have something else in your mind?

[reyang]

Yes, a simple lock should solve the problem.