[TEST] Fix possible race condition in checksum name generator

When three threads are trying to write checksums at the same time, it's possible for all three threads to obtain the same checksum file name A. Then the first thread enters the synchronized section, creates the file with name A and exits. The second thread enters the synchronized section, checks that A exists, creates file A+1 and exits the critical section. Then it proceeds to clean up  and deletes all checksum files including A. If it happens before the third thread enters the synchronized section, it's possible for the third thread to check for A and since it no longer exists create the checksum file A the second time, which triggers "file _checksums-XXXXXXXXXXXXX was already written to" exception in MockDirectoryWrapper and fails recovery.
[INDEX] Ensure `index.version.created` is consistent

Today `index.version.created` depends on the version of the master
node in the cluster. This is potentially causing new features to be
expected on shards that didn't exist when the index was created.
There is no notion of `where was the shard allocated first` such that
`index.version.created` can't be reliably used as a feature flag.

With this change the `index.version.created` can be reliably used to
determin the smallest nodes version at the point in time when the index
was created. This means we can safely use certain features that would
for instance require reindeing and / or would not work if not the
entire index (all shards and segments) have been created with a certain
version or newer.

Closes #6660
[TEST] Prevent usage of System Properties in the InternalTestCluster

All settings should be passes as settings and the enviroment should not
influence the test cluster settings. The settings we care about ie.
`es.node.mode` and `es.logger.level` should be passed via settings.
This allows tests to override these settings if they for instance need
`network` transport to operate at all.

Closes #6663