Add a few more details to the cluster client docs.
Merge branch 'master' into more_cluster_client_docs
=act Fix typo in scheduler error message
=doc #3539 Update the cluster spec

* The cluster spec has been rearranged into two sections describing the current cluster and future enhancements/additions.
* The state diagram has been replaced with a new one describing the current cluster implementation.
* A number of small additons and changes to the text.
=doc #3500 Add section about Dependency Injection
=cam #3541 Log Camel endpoint activation/deactivation failures as error
=rem #3527 Correction of match in reWatch
+rem #3504: Toned down error logging and made loglevel configurable
Merge pull request #1654 from taylorleese/more_cluster_client_docs

=con Add a few more details to the cluster client docs.
Merge pull request #1669 from oakwhiz/master

=act Fix typo in scheduler error message
Merge pull request #1677 from akka/wip-3500-doc-di-patriknw

=doc #3500 Add section about Dependency Injection
Merge pull request #1678 from akka/wip-3527-wrong-match-patriknw

=rem #3527 Correction of match in reWatch
=act #3525 Retry finishConnect after OP_CONNECT

Patch picked from https://github.com/spray/spray/commit/63648f17807035b4c17b5377ec67f5455a31e5a6
Merge pull request #1681 from drewhk/wip-3504-tone-down-remote-logging-drewhk

+rem #3504: Toned down error logging and made loglevel configurable
Merge pull request #1675 from akka/wip-finish-connect-retries-patriknw

=act #3525 Retry finishConnect after OP_CONNECT
=act, rem, clu #3521: make serialize-messages work with core modules
+act #3529 Add convenience helper for looking up an actor by identity

* ActorSelection.resolveOne
Merge pull request #1671 from akka/wip-3529-identify-sugar-patriknw

+act #3529 Add convenience helper for looking up an actor by identity
=act #3389 Improve SSL closing sequence

* Port of this fix in Spray by @jrudolph from this pull request https://github.com/spray/spray/pull/400
Merge pull request #1673 from akka/wip-3541-better-logging-of-camel-activation-failures-ban

=cam #3541 Log Camel endpoint activation/deactivation failures as error