Fix Gitaly crashing without a WAL archiver configured

Gitaly is currently crashing if a WAL archiver isn't configured as soon as a partition is initialized on access. This is because we're defining a nil *LogEntryArchiver and passing it to NewPartitionManager as a LogConsumer interface instance. This leads to failing nil checks as the LogConsumer interface instance now has its type field set to *LogEntryArchiver. Fix this by instead defining the possible archiver as the interface type so it remains nil even if we don't initialize it with an actual WAL archiver.