Fix rare NPE caused by batchlog replay / node decomission races

Batchlog replay process collects addresses of the hosts that have been hinted to, so it can flush hints for them to disk before confirming deletion of the replayed batches. If a node has been decommissioned during replay, however, when the time comes to flush the hints at the very end of replay, StorageService.getHostIdForEndpoint() will return null for its address, which will, down the line, cause HintsCatalog::get() to be invoked with a null host id argument, causing an NPE.

The simple fix is to check returned host ids for addresses for nulls, and collect hinted host ids instead of hinted addresses.