CouchKVStoreFileCache limit on cluster_run likely to prevent full bucket of DCP streams

The CouchKVStoreFileCache takes it's limit from the available number of file descriptors to the process (set via launchctl on macOS). We currently divide the file descriptors given to the engines by up to 3 (depending on how many backend processes are running). Whilst we have a minimum value for the engine file descriptors of 1050 (1024 for backfills + some extra for flushing, reading, logging etc) when we divide this limit for each supported backend we run into issues with cluster_run as we can't do a full stream of a bucket. Normal installations should attempt to set the file descriptor limit to 200,000 which should be enough to avoid these problems.

We need the minimum limit for a CouchKVStoreFileCache to be at least 1024 to support basic functionality in cluster_run.