Ensure a minimum retry interval of 5 seconds in fetching central configuration

# Description
The central config spec [has recently been updated](https://github.com/elastic/apm/pull/667/files) to say that regardless of what the Cache-Control headers are, agents should only retry at a maximum every 5 seconds to fetch the central configuration. I.e. if the Cache-Control headers specify a value less than 5, the default retry interval should be used.

## Agent Issues
- [ ] https://github.com/elastic/apm-agent-java/issues/2803
- [ ] https://github.com/elastic/apm-agent-dotnet/issues/1831
- [ ] https://github.com/elastic/apm-agent-nodejs/issues/2941
- [ ] https://github.com/elastic/apm-agent-python/issues/1645
- [ ] https://github.com/elastic/apm-agent-go/issues/1322
- [ ] https://github.com/elastic/apm-agent-php/issues/778