Summary:
Currently hazard pointers doesn't support stealing any of the pointer bits.
You can *almost* roll it yourself using try_protect, but this prevents
implementations from choosing their type of barrier.

This adds a new get_protected interface that you can use to steal bits, or
otherwise manipulate pointers as you would like.

This also adds a MWMR list based set example that uses it, that is wait-free
for readers (unlike the SWMR example, that is only lock-free).

Reviewed By: magedm

Differential Revision: D5455615

fbshipit-source-id: 53d282eda433e00b6b53cd804d4e1c32c74c2fb8