add version info to t=JSON output data

[warner]

It will probably make life easier in the future if we add a version number to the JSON output from operations like deep-stats, deep-manifest, and deep-check. A related piece would be to tell developers that they should tolerate the presence or absence of each field. On the other hand, the only customers of the t=JSON are 1) the Tahoe CLI, and 2) tools written to use the webapi functions, and I'm ok with requiring that you use e.g. a 1.3.0 CLI with a 1.3.x backend instead of a 1.4.x backend, and developers have to play catchup all the time anyways. So I'm not sure how much energy I want to put into this.