Core: put []'s around the values in the slow-merge log message
[TEST] Use private store for a test private engine

The test was using the same store as the suite level engine which caused
problems with write locks in some cases.

Closes #8880
[CORE] Drop support for state written pre 0.90

Today we have several upgrade methods that can read state written
pre 0.90 or even pre 0.19. Version 2.0 should not support these state
formats. Users of these version should upgrade to a 1.x or 0.90.x version
first.

Closes #8850
[TEST] wait for yellow to avoid searching while relocating

After upgrading shard might start relocating again. If there are no
replicas the cluster state of a node might not be up to data for
a few miliseconds and direct a search request to a node that does not
have the shard anymore. This result in the following test failures:

1> java.lang.AssertionError: Count is 99 but 101 was expected.  Total shards: 13 Successful shards: 12 & 0 shard failures:
1>     __randomizedtesting.SeedInfo.seed([1932F73B458703CA:6F4FAD3DAC55591C]:0)
1>     [...org.junit.*]
1>     org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertHitCount(ElasticsearchAssertions.java:184)
1>     org.elasticsearch.bwcompat.BasicBackwardsCompatibilityTest.testIndexRollingUpgrade(BasicBackwardsCompatibilityTest.java:358)

Waiting for relocation finished should fix this.
Docs: use https to download the gpg public key

Closes #8818