Fix premature shutdown of HTTP connection pool

[oguzhanunlu]

S3 Loader 2.0.0 throws java.lang.IllegalStateException: Connection pool shut down after a RecordProcessor hits end of a shard.

Behind the scenes, reaching the end of a shard causes shutdown of its' RecordProcessor which in turn shutdowns the emitter of that record processor. Shutting down the emitter calls client.shutdown where client is shared across all emitters. Given that an S3 Client maintains its own connection pool, shutting down the single S3 client shutdowns its' connection pool as well.

We should not be shutting down the S3 client at KinesisS3Emitter.shutdown which is a premature shutdown.