netbox: reuse _request() to do SQL execute()

_request() is a wrapper for perform_request, that detects schema
changes, and waits until it is reloaded. Lets use _request()
instead of direct perform_request() for execute(). The reason why
the _request() was not used earlier was my attempt to avoid
multiple return values in _request(), that leads to minor fixes
in non-execute methods like index.select or eval/call_16, which
return the _request() directly.

But diplicating schema reloading logic for execute() is worse.

Closes #3323
Closes #3322