Add retry logic for S3 connection errors when restoring snapshots

This commit adds a retry logic when reading blobs from S3. It also adds a retry logic when initializing a multipart upload and sets the internal "max retries" parameter of the Amazon S3 client with the same value as the "max_retry" parameter set for the snapshot repository (so in worst cases with the default value set to 3, 3x3=9 attempts will be made). The internal S3 client uses an exponential back off strategy between each connection exception (mainly IOException).

Closes elasticsearch/elasticsearch#8280
Closes #140.
AwsEc2UnicastHostsProvider should use version.minimumCompatibilityVersion()

The AwsEc2UnicastHostsProvider creates DiscoveryNodes that are used as an initial seed for unicast based discovery. At the moment it uses Version.CURRENT (see [1]) for those DiscoveryNode object, which confuses the backwards compatibility layer to think this nodes are of the latest version. This causes new nodes to fail to join old nodes as the ping serialization goes wrong. Instead we should use version.minimumCompatibilityVersion(). See [2]

[1] https://github.com/elasticsearch/elasticsearch-cloud-aws/blob/es-1.x/src/main/java/org/elasticsearch/discovery/ec2/AwsEc2UnicastHostsProvider.java#L165
[2] https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/discovery/zen/ping/unicast/UnicastZenPing.java#L130

Ps. this was reported on the mailing list. See: https://groups.google.com/forum/#!msg/elasticsearch/8pUwFld88tI/7jRuG6hqtbAJ

Closes #143.