src: fix up smalloc weak persistent usage

Fix a regression that was introduced in commit ce04c726 after the
upgrade to V8 3.24.

The new weak persistent handle API no longer gives you the original
persistent but still requires that you clear it inside your weak
callback.

Rearrange the code in src/smalloc.cc to keep track of the persistent
handle with the least amount of pain and try hard to share as much
code as possible between the 'just free it' and 'invoke my callback'
versions of the smalloc API.

Fixes #7309.