IOBuf: add a method to signal the underlying buffer as externally shared

IOBuf: add a method to signal the underlying buffer as externally shared

Summary:
There are use cases where 1). the underlying buffer is externally managed (e.g. by a slab allocator) and 2). we need to do bookkeeping when the wrapped IOBuf gets destroyed (e.g. reference counting). This diff adds a another method to mark the underlying buffer as shared with the external memory management mechanism.

The `takeOwnership` doesn't meet the criteria since it assumes the ownership of the buffer, while in this case we need to signal it as externally managed so that hopefully callers won't try to modify the underlying buffer.

Reviewed By: simpkins

Differential Revision: D2662954

fbshipit-source-id: e908c3ebeeefe9a5d332c75070f377fb1dad5acb