Feature request: don't make API requests if a valid cache is present

Currently, enabling `cacheFlags` still causes API requests to be made unless `cacheOptions.skipAPI` is also enabled. The intention is to speed up the initial load while updating flags in the background.

IMO this is not a reasonable default. Every cache implementation I've worked with will use the cache instead of calling the data source - this is exactly the goal of using any cache. One common reason for enabling caching is to reduce the number of API calls made, which does not actually happen when caching is enabled unless you also enable `cacheOptions.skipAPI`.

We should make `cacheOptions.skipAPI: true` the default behaviour and remove this option. If users want to repopulate their flags even when using caching, they should have the option to do that by manually calling a method such as `getFlags`.