Fix trimStart to follow the chain
Fix trimStart to follow the chain

Summary:
trimStart only works on the current IOBuf. It should follow the chain
if the current IOBuf's length is shorter than needed.

Reviewed By: tudor, dihde

Differential Revision: D2871342

fb-gh-sync-id: 7e01f56444747e9d0cededfe38c1e092575aec1a

LinusU/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,LinusU/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,facebook/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,facebook/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,facebook/fbthrift,facebook/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift