…sabled

Details of the issue as well as local reproducers are documented in 3scale#1467.

This change ensures that when `boot.init_worker` checks to see whether or not
the configuration needs to be reloaded, it also checks to confirm that the `boot.ttl()`
value set from `APICAST_CONFIGURATION_CACHE` is a positive number, in order to avoid
an immediate and infinite recursive loop of refreshing the configuration.

This should still preserve the initial fix [1] where this regression appears to
have been introduced, as we should never schedule reloading with the configuration settings of:

```
APICAST_CONFIGURATION_LOADER: boot
APICAST_CONFIGURATION_CACHE: -1
```

We should also never have a situation where we'd need to account for `interval = 0`
since we fail out in `boot.init` in that case [2].

Tested locally as documented in the issue, however based on the comment in the initial
fix PR, I've not added any tests in code. Happy to do so if there's now a good way
to accomplish that!

[1] 3scale#1399
[2] https://github.com/3scale/APIcast/blob/7e7eaf6f1d584c78d999c0d09f5b65203161c402/gateway/src/apicast/configuration_loader.lua#L146-L149

Simplifies the conditional to reload configuration based on the interval set by
`APICAST_CONFIGURATION_CACHE`.

Also conditionally sets the interval to schedule.