Add DCHECKs for checking that underlying IOBuf wasn't modified
Add DCHECKs for checking that underlying IOBuf wasn't modified

Summary: Appending/prepending to IOBuf while iterating over it with Cursor is unsafe. This diff adds DCHECKs to catch such cases.

Reviewed By: yfeldblum

Differential Revision: D6735060

fbshipit-source-id: 7799facc52c53fabd83756ecb26a18c4ebd69677

facebook/folly,facebook/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly