add /status?t=json

[warner]

The allmydata native client needs to be able to get some progress information out of its local tahoe node (so it can provide feedback to the user), and the easiest way looks to be adding a machine-readable format to the /status page. This page describes the currently-active upload/download/publish/retrieve/mapupdate operations, with progress information and detailed results (which shares were put where, timing information, etc). It has similar information for the last 20-ish operations in each category: those which have completed recently.

The proposal is to add /status?t=json, and have it report just the current in-progress operations in a machine-readable table. This would be enough for the native client to provide at least *some* form of progress data to the user.

Later, we can add "operation markers": a random string, provided by the client in a query argument to the PUT or GET webapi method. This string would then be added to the /status?t=json output, so the client could correlate the two.

[warner]

ok, /status/?t=json is done. No operation markers. Docs in source:docs/webapi.txt#L750