Summary:
Flipper's network plugin may cause crash on a frequency of about one time per 1~2 days(I think it depends on how many network request we send).

I assume the crash is caused by part of the code of HTTPParser is not thread-safe, and invoke it asynchronously may cause double-free crash. So I manually ask for HTTPBody synchronously, make the HTTPParser parse the request and cache them in advance, before any possible asynchronous invoking.
## Changelog
Fix potential crash cause by network plugin.
Pull Request resolved: https://github.com/facebook/flipper/pull/563
Test Plan:
1. Run our App with Flipper integrated
2. Make sure FlipperNetworkPlugin is active
3. It may take days to reproduce the crash
Reviewed By: passy
Differential Revision: D17739327
Pulled By: priteshrnandgaonkar
fbshipit-source-id: e814145c346bd2da1d2f5f87b6a2f7e200f0bf2d