Support batched creation of Captchas

[hrj]

For providers such as LabelCaptcha, a set of generated Captchas might be only valid for the state of the system at generation time. As the state changes, for example when the set of known and unknown words changes, the older Captchas might become irrelevant.

So, to address this, I propose creating Captchas in batches. For example, one possible design could be:

• When running low on Captchas, a Captcha provider which has no unsolved captchas is picked for generating more captchas.
• A NewBatch message is sent to the provider. In its handler, the provider might update its state, for example, reclassify known and uknown words.
• the CreateChallenge message is then sent N times to this provider, where N is configurable