Add workaround for expiring signer token

[tobiasbayer]

Fixes #7

This is the workaround for expiring tokens suggested in aws/aws-sdk-ruby#961

[malpani]

Thank you for the change but this will not work as it will create signer with the same credentials. Trevors fix indicates creating new credentials and then using them to create a new signer. https://github.com/awslabs/logstash-output-amazon_es/blob/master/lib/logstash/outputs/amazon_es/aws_transport.rb#L68

I will use your pull request as baseline and make changes for the same.

[tobiasbayer]

Strangely, this fix actually does work with our setup. I think the problem is that using the same signer over and over again reuses a cached token which then expires. Creating a new signer forces the retrieval of a new token.

[malpani]

ahh I see! Thanks, in that case pulling this in as it is anyway backwards compatible. There will be additional signer objects created even for the hardcoded access/secret credential case which can be optimized later if needed.