Fixed heap-use-after-free in js_set handler.

Fixed heap-use-after-free in js_set handler.

The issue was introduced in commit 04f6dfb (0.9.2) by moving VM
destruction from the pool cleanup handler to the http cleanup handler.

Moving VM destruction to the http cleanup handler broke js_set variable
usage during the log phase, because these variables are called after the
VM has been destroyed.

The fix is to move VM destruction back to the pool cleanup handler, but
use a temporary pool while njs.on('exit', ...) is executing.

This fixes #969 and #971 issues on Github.