=doc Move up dependency info for DurableMailboxSpec
=act #3547 Resolve high contention on unsubscribeAddresssTermination
=doc Correct typo in cluster docs
Merge pull request #1639 from akka/wip-durable-mb-doc-∂π

=doc Move up dependency info for DurableMailboxSpec
Merge pull request #1641 from joa/patch-1

Update cluster.rst
Merge pull request #1643 from octo47/wip-3542-no-defensive-unsubscribe

Resolves high contention on unsubscribeAddresssTermination #3547
Merge remote-tracking branch: upstream/master