possible tight infinite loop of central config requests

[trentm]

elastic/apm-nodejs-http-client#185 was recently implemented for #2941
Ironically it made things worse.

Typically APM server's endpoint for central config will return a cache-control header, e.g.:

Cache-Control: max-age=<number of seconds>

If so, everything with v3.40.0 of the APM agent is fine.

However, if a central config request gets a response that doesn't specify a Cache-Control header, or one without a 'max-age' field, then getMaxAge(res) returns NaN and getCentralConfigIntervalS(NaN) unintentionally returns NaN:

> const INTERVAL_MIN_S = 5
undefined
> const INTERVAL_MAX_S = 86400 // 1d
undefined
> seconds = NaN
NaN
> Math.min(Math.max(seconds, INTERVAL_MIN_S), INTERVAL_MAX_S)
NaN

which leads to a tight loop of the agent hammering APM server for central config.