add cancelAllConfirmations method

[srg-n]

cancelAllConfirmations(time, confKey, cancelKey, callback)

• time - Should be a unix timestamp
• confKey - Should be a base64-encoded key for the "conf" tag, generated using the supplied time
• cancelKey - Should be a base64-encoded key for the "cancel" tag, generated using the supplied time
• callback - Called when the request completes
• err - An Error object on failure, or null on success
• confs - An array of the confirmations that were just cancelled. May be empty if there were no pending confirmations.

Cancel all outstanding confirmations on your account all at once.

[DoctorMcKay]

This can already be done in userland code and it's not exactly a common function; for the sake of simplicity I'm not sure it's worth adding a dedicated method for this.