Verson bump to 8 on master

[alpar-t]

Contains changes in #38513 as well.

[DaveCTurner]

Just a heads-up that there's a booby-trap here:

elasticsearch/server/src/main/java/org/elasticsearch/cluster/coordination/DiscoveryUpgradeService.java

Line 108 in a5c35f9

assert Version.CURRENT.major == Version.V_6_6_0.major + 1 : "remove this service once unsafe upgrades are no longer needed";

I am working on a commit that deletes the offending code.

[DaveCTurner]

When you're ready, you can cherry-pick 1898ee4776c541e0c2a5b4428f64aaa5dacb8164 which removes the Zen1 BWC infrastructure that this assertion asserts to be absent. You may need to fetch that from my fork: https://github.com/DaveCTurner/elasticsearch/tree/2019-02-06-remove-zen1-bwc

[jasontedor]

I ported all these commits over to #38514. Thanks all for the contributions, let's continue the effort there.