Don't retry replications for shard/node pairs no longer in shardmap

mem3_sync will continuously attempt to replicate shards for nodes that are down,
even if those nodes were removed from the shard map. This leads to an infinite
loop of retries for nodes that have been decomissioned from the cluster.

BugzID: 39298
Merge pull request #76 from cloudant/39298-dead-replication-retries

Don't retry replications for shard/node pairs no longer in shardmap