invalid values for time/duration config vars gives surprising and unwanted behaviour

There are a few config vars that support a time/duration as a string of the form ``, e.g. "10s" to mean 10 seconds. The supported units are "m" for minutes, "ms" for milliseconds, "s" for seconds. If the config value is a number or a string without units, it defaults to seconds.

Currently that parsing/conversion is done by `normalizeTime` in lib/config.js:

```
function normalizeTime (opts) {
for (const key of TIME_OPTS) {
if (key in opts) opts[key] = toSeconds(String(opts[key]))
}
}
```

and the relevant config vars are:

```
var TIME_OPTS = [
'abortedErrorThreshold',
'apiRequestTime',
'metricsInterval',
'serverTimeout',
'spanFramesMinDuration'
]
```

Currently `toSeconds()` above will return *`null`* if the string value is invalid (e.g. "10ss", "10h", "fuzzywuzzy"). There is no log warning. The results can be surprising: If this `null` is used in math (often that is the case as the seconds value is converted to milliseconds for usage in `setTimeout` and the like) or comparisons, JavaScript will treat it as zero, which sometimes is a sentinel value that means something unexpected. Specifically:

- `apiRequestTime`: Results in a value of `0` being used to the http-client ->
a `-1` sentinel value to StreamChopper, which results in time-based closing
of requests to APM server *not* happening which -- for low traffic apps --
results in `ERROR (elastic-apm-node): APM Server transport error: APM Server response timeout (30000ms)`.
which results in the stream chopper not
- `metricsInterval`: Results in null -> 0 -> turning *off* metrics collection.
- `serverTimeout`: Results in no socket timeout being set on the communication
with APM server which could rarely result in hung communication with APM server.
- `spanFramesMinDuration`: Results in possibly unexpected additional CPU usage
as the agent collections stack traces for all spans.
- `abortedErrorThreshold`: Not yet investigated. Comment indicate this is an
old undocumented(?) feature that "will be removed".