Add support for saving / restoring shared memory.

Summary: Subsequent commits will hook this up to a command-line option.

Reviewed By: @jwatzman

Differential Revision: D1610700
Add ability to save and restore server state.

Reviewed By: @jwatzman

Differential Revision: D1610805
Simplify Program.serve()

Summary: `report` is never modified in the current code, so there is no way
the panic can get triggered.

Reviewed By: @jwatzman, @elgenie

Differential Revision: D1613871