Making the AmazonS3Client instance a member field to be reused  ...

... throughout the lifetime of the S3FileSystem instance.

The problem creating a new instance of AmazonS3Client for every action on the S3FileSystem instance is that each time you need to establish a new SSL session to S3 with every new connection, and we found this to be very determental to performance. Thus, keeping the instance cached elimates the SSL (re-)negotation, and uploads can proceed faster.