Avoid saving global storage section.

Summary: I took a closer look at the code and realized that the 1GB of global
storage is always reset after each typechecking phase. (The memory isn't
zeroed out, but it is marked as unused.)

So this commit moves the global storage section to the front of the shared
memory (instead of in the middle), and hh_server only saves everything that
comes after that.

Reviewed By: @elgenie

Differential Revision: D1683934

Signature: t1:1683934:1416280066:0567f4d4ec45d72b4670851a51f413b693fdf3ce
Fix session GC probability check

Summary: Not entirely sure if this is the right fix but it was proposed in the discussion
on the issue.

Fixes #3747

Reviewed By: @alexmalyshev

Differential Revision: D1681222

Signature: t1:1681222:1415941286:c2ea5202963788b7b921783cd69dd93f775e10f9