Rethink debug lifecycle

[syntactically]

Currently, the initial breakpoint set up when creating a sandbox is set on the entrypoint function, which means that, when creating a sandbox from an initialised snapshot, it is never set up or executed, and a debugger has no easy way to attach.

Relatedly, the debugging infrastructure (and the initial breakpoint) is/are set up in Sandbox::new(), which means that whether a sandbox has a debug thread or not is decided once, at sandbox creation time, but there is also only one place to attach a debugger (at sandbox initialisation time): future function calls, or restored sandboxes, might not be easy to debug, even though they have a debug thread consuming resources.

See also: this comment

[ludfjig]

Note that #1120 might further change debugging due to resetting debug register on snapshot-restores, which could potentially discard breakpoints etc. One possible solution might be to not reset debug registers if gdb cargo feature is enabled?