Fix a corner case of use-after-free with never started profiler

[szegedi]

What does this PR do?:
Moves GC callback registration from WallProfiler constructor to StartImpl, to be symmetrical with their deregistration through StopImpl->Dispose.

Motivation:
While we don't do this right now, it's possible to create a profiler and not start it. In that case, it will also never be stopped, and a GC callback that is installed can be invoked on a deleted profiler causing a use-after-free. With this change, the callbacks are only registered when the profiler is started, so their lifecycle follows that of start/stop.

Additional Notes:
This was broken out of #255 as a simple small fix. It comes up sometimes with ASAN especially when a test fails when we're trying to update dependencies.

[github-actions]

Overall package size

Self size: 1.77 MB
Deduped: 2.16 MB
No deduping: 2.16 MB

Dependency sizes

| name | version | self size | total size | |------|---------|-----------|------------| | source-map | 0.7.6 | 185.63 kB | 185.63 kB | | pprof-format | 2.2.1 | 163.06 kB | 163.06 kB | | p-limit | 3.1.0 | 7.75 kB | 13.78 kB | | delay | 5.0.0 | 11.17 kB | 11.17 kB | | node-gyp-build | 3.9.0 | 8.81 kB | 8.81 kB |

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}