Don't leak HashContexts

Summary: HashContext is allocated via the global operator new,
and it contains a std::shared_ptr, so we need the default
sweep behavior, which is "delete this".

Reviewed By: @bertmaher

Differential Revision: D1668713

Signature: t1:1668713:1415417867:13da80e72cce887104b547e737fbbd37fcbd52cc