Add support to decode a resume token

Adding a new subcommand to zstream called token. This
now allows users to decode a resume token to retrieve the toname
field. This can be useful for tools that need this information.
The syntax works as follows zstream token <resume_token>.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Signed-off-by: Tony Perkins <tperkins@datto.com>
Closes #10558