Move from structopt to clap

[nitisht]

structopt is in maintenance mode now as clap has all the features now.

Ref: https://github.com/TeXitoi/structopt#maintenance

Accordingly we need to move to clap now. Also there are issue currently on how the help message is printed:

➜ ./parseable --help
S3 config 0.0.3
configuration for AWS S3 SDK

USAGE:
    parseable [OPTIONS]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
        --s3-access-key-id <s3-access-key-id>
            The access key for AWS S3 or compatible object storage platform [env: P_S3_ACCESS_KEY=]  [default:
            minioadmin]
        --s3-bucket-name <s3-bucket-name>
            The AWS S3 or compatible object storage bucket to be used for storage [env: P_S3_BUCKET=]  [default:
            parseable]
        --s3-default-region <s3-default-region>
            The region for AWS S3 or compatible object storage platform [env: P_S3_REGION=]  [default: us-east-1]

        --s3-endpoint-url <s3-endpoint-url>
            The endpoint to AWS S3 or compatible object storage platform [env: P_S3_URL=]  [default:
            https://minio.parseable.io:9000]
        --s3-secret-key <s3-secret-key>
            The secret key for AWS S3 or compatible object storage platform [env: P_S3_SECRET_KEY=]  [default:
            minioadmin]

Notice the description says

S3 config 0.0.3
configuration for AWS S3 SDK

Also several other fields are not printed here in the help message.