Purge store if there is another node already running

Preloading happens before communicating with other nodes that might already have the cache running. When joining the existing members, the cache then waits to receive the first CH in which it is a member, and then deletes only the entries in the segments that it doesn't own in that CH.

The intention of this was to remove as little as possible from the existing data, e.g. if the first node to start up is not the one that was stopped last. But the preloaded entries are not replicated to the other nodes, so this can lead to inconsistencies.

It would be better to delay preloading until we know we are the first node to start up, but failing that we could clear the data container and the store before receiving the initial state.

Note that this will only allow preloading data from one node. Restoring data from more nodes is harder to do, and we will implement it as part of graceful restart.