add Cursor::isAtEnd()

add Cursor::isAtEnd()

Summary:
Add a helper method to efficiently check if the cursor is at the end of the
IOBuf chain.  This is equivalent to (cursor.totalLength() > 0), but it doesn't
need to walk the entire chain just to tell if it is at the end or not.

Test Plan: Updated the unit tests to contain some checks for isAtEnd().

Reviewed By: jasmeetbagga@fb.com

Subscribers: trunkagent, doug, net-systems@, exa, folly-diffs@, yfeldblum

FB internal diff: D1875345

Signature: t1:1875345:1425006512:49ac246fd0ac7937fdcd6cf1359a841f048c444e