Core: don't replace indices within the ActionRequest with the concrete ones, and make sure check blocks is executed on concrete indices

Concrete indices is now called multiple times when needed instead of changing what's inside the incoming request with the concrete indices. Ideally we want to keep the original aliases or indices or wildcard expressions in the request.

Also made sure that the check blocks is done against the concrete indices, which wasn't the case for delete index, delete mapping, open index, close index, types exists and indices exists.

Closes #6694
Closes #6777
[Discovery] verify connect when sending a rejoin cluster request

When a master receives a cluster state from another node, it compares the local cluster state with the one it got. If the local one has a higher version, it sends a JoinClusterRequest to the other master to tell it step down. Because our network layer is asymmetric, we need to make sure we're connected before sending.

Closes #6779
ZenUnicastDiscoveryTest works again.