LookupCache shouldn't call entryCount() before cache was started

`LookupCache` should call `LookupCache#entryCount()` before the cache was actually started.

When using a remote data source which isn't available until `LookupCache#doStart()` has been called, `LookupCache#entryCount()` might not return a sensible value or even throw an exception (e. g. because the connection to the remote data source hasn't been established yet).

https://github.com/Graylog2/graylog2-server/blob/2c418975989da2fff589e35cdf8c00ff41983e52/graylog2-server/src/main/java/org/graylog2/plugin/lookup/LookupCache.java#L63

## Expected Behavior

Don't call `LookupCache#entryCount()` before cache has been initialized via `LookupCache#doStart()`.

## Current Behavior

The `LookupCache` constructor calls `LookupCache#entryCount()` before `LookupCache#doStart()` has been executed.