Boaz Leskes 5e7d22357f Connect to new nodes concurrently (#22984)
When a node receives a new cluster state from the master, it opens up connections to any new node in the cluster state. That has always been done serially on the cluster state thread but it has been a long standing TODO to do this concurrently, which is done by this PR.

This is spin off of #22828, where an extra handshake is done whenever connecting to a node, which may slow down connecting. Also, the handshake is done in a blocking fashion which triggers assertions w.r.t blocking requests on the cluster state thread. Instead of adding an exception, I opted to implement concurrent connections which both side steps the assertion and compensates for the extra handshake.
2017-02-06 16:32:41 +01:00
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