Add SQS Async Client support

[KL0nLutiy]

Issue #187 :

Description of changes:
Add basic support of SqsAsyncClient:

1. Create AmazonSQSMessagingClient interface that is implemented by old AmazonSQSMessagingClientWrapper and new AmazonSQSAsyncMessagingClientWrapper classes. AmazonSQSAsyncMessagingClientWrapper is wrapper for SqsAsyncClient
2. Create AbstractSQSConnectionFactory abstract class that is used by old SQSConnectionFactory and new SQSAsyncConnectionFactory classes. SQSAsyncConnectionFactory create SQSConnection that uses SqsAsyncClient
3. Update all classes to use AmazonSQSMessagingClient interface

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[KL0nLutiy]

@ziyanli-amazon, please take a look on PR

[bjhartin]

This would allow folks using gatling to load test sqs apps with the async client. We could use this!