=act #3602 report `Connected` only when connection attempt was successful

Before this change, a client connection was always instantly reported as
`Connected`, even if the endpoint would never respond at all.

The reason is a weird behavior of OP_CONNECT and SocketChannel in the
JDK (observed in Linux):
 - a channel is always connectable before the connection is attempted
   (`channel.connect`). Selecting for OP_CONNECT before the `connect`
   call will instantly report connectable
 - even worse: after OP_CONNECT was reported true, also `finishConnect`
   will always return true, even if the connection wasn't yet established.
   That's probably the case because `finishConnect` is internally implemented
   depending on previous epoll results (on Linux).
small spelling fix
=act #3645 Make TcpOutgoingConnection actor handle UnresolvedAddressExceptions
#3655 clu Stop sending EndHeartbeat after ack
!rem #3646: Optimize sending of Ack messages
Merge pull request #1754 from drewhk/wip-3646-sysmsg-ack-optimization-drewhk

!rem #3646: Optimize sending of Ack messages
Merge pull request #1759 from akka/wip-3655-stop-endheartbeat-patriknw

#3655 clu Stop sending EndHeartbeat after ack
Merge pull request #1732 from luc-at-jr/master

small spelling fix in Slf4jEventHandler.scala
!per #3652 Custom snapshot and persistent message serialization

- Protobuf serializer for Persistent message
- Configurable serializer for Persistent message's payload
- Configurable serializer for snapshots
+clu #3636 Revert join to uninitialized

* Revert the change introduced in
  https://github.com/akka/akka/pull/1738/files
* The cleanup/improvements aside of the actual
  feature is not reverted by this patch
* Clarify the documentation
Merge pull request #1757 from eligosource/wip-3652-custom-serialization-krasserm

!per #3652 Custom snapshot and persistent message serialization
=clu #3660 Simple speedup of gossip in early phase
Merge pull request #1764 from akka/wip-3660-simple-speedup-of-gossip-in-early-phase-ban

=clu #3660 Simple speedup of gossip in early phase
Merge pull request #1752 from spray/wip-3645-handle-UnresolvedAddressExceptions

=act #3645 Make TcpOutgoingConnection actor handle UnresolvedAddressExceptions
=doc #3639 Show complete custom mailbox example.
Merge pull request #1768 from akka/wip-3639-doc-show-producesmessagequeue-ban

(forward port) =doc #3639 Show complete custom mailbox example.
Merge pull request #1765 from akka/wip-3636-revert-join-uninitialized-patriknw

+clu #3636 Revert join to uninitialized
!act,rem,clu #3549 Simplify and enhance routers

* Separate routing logic, to be usable stand alone, e.g. in actors
* Simplify RouterConfig, only a factory
* Move reading of config from Deployer to the RouterConfig
* Distiction between Pool and Group router types
* Remove usage of actorFor, use ActorSelection
* Management messages to add and remove routees
* Simplify the internals of RoutedActorCell & co
* Move resize specific code to separate RoutedActorCell subclass
* Change resizer api to only return capacity change
* Resizer only allowed together with Pool
* Re-implement all routers, and keep old api during deprecation phase
* Replace ClusterRouterConfig, deprecation
* Rewrite documentation
* Migration guide
* Also includes related ticket:
  +act #3087 Create nicer Props factories for RouterConfig
Merge pull request #1746 from akka/wip-3549-shiny-routers-patriknw

!act,rem,clu #3549 Simplify and enhance routers
+clu #3627 Cluster router group  with multiple paths per node

* Use the ordinary routees.paths config property instead of
  cluster.routees-path
* Backwards compatible in deprecation phase