Make transaction IDs more accessible

[lukechampine]

DELETE /limbo/:txid requires you to specify the ID of the transaction, but the ID isn't present in the response of /limbo, which means you either need to track it independently, or recompute the ID using the transaction object (which is rather difficult if you're not using Go).

PUT /limbo/:txid suffers from the same problem, although I suppose we could alleviate it by switching the endpoint to POST /limbo.

The most generic solution would be a /txid method that computes and returns the ID of the transaction supplied in the request body. But I dunno, it feels like overkill to add an endpoint that just calls a method on the provided object.

[lukechampine]

Another option would be for /limbo to reply with a list of txids, rather than the transactions themselves. This would be more consistent with /transactions. The downside is that you pretty much always want to see the whole limbo set, so this would turn a single request into a bunch of requests.

[lukechampine]

Adding on to this, /broadcast should return an array of transaction IDs.