Secondaries should not forward replication information for removed chained nodes

ISSUE SUMMARY
On a replicaset that uses chained replication, if a secondary with id M that syncs from secondary with id N is removed, node N continues to forward replication information about M to the primary.

USER IMPACT
The following message appears repeatedly in the primary's logfile:

replset couldn't find a slave with id M

If the removed node is required to meet a specific write concern, write operations with that write concern will wait indefintely unless a wtimeout was specified.

On a sharded cluster, during a chunk migration the destination shard will wait for the final writes to be replicated to the majority of the nodes. If the write concern cannot be satisfied and a wtimeout was not specified, the chunk migration times out after 60 minutes.

WORKAROUNDS
There is no workaround for this issue.

AFFECTED VERSIONS
MongoDB 2.6 versions up to 2.6.5 are affected by this issue.

FIX VERSION
The fix is included in the 2.6.6 production release.

RESOLUTION DETAILS
Secondaries no longer forward replication progress for nodes that are no longer part of a replica set