Summary:
There exists race condition in `sendRequest:withDelegate:` method, it can do the session creation multiple times, because we don't lock that, which would leads `EXC_BAD_ACCESS` because use and deallocated session concurrently, we can refer to how to create a singleton safely.
Related https://github.com/facebook/react-native/issues/25152.
## Changelog
[iOS] [Fixed] - Fixes race condition of Network module
Pull Request resolved: https://github.com/facebook/react-native/pull/25156
Differential Revision: D15671734
Pulled By: sammy-SC
fbshipit-source-id: 5021e6cf33c2b55e3f7adf573ab5c8e6a8d82e23