[feature] APIs which charge should allow price limiting

[happybeing]

There can be large variations between the prices charged for similar operations, including between estimate and execution cost for a given operation.

This makes it necessary for client software to limit what may be deducted from a wallet during any API call that may do this.

This will be especially important during the early phase of a live network where bugs could drain a large sum if limits are not supported, and would be a PR disaster. A recent bug in the transaction code that was paying nodes massive rewards demonstrates that a critical bug could still be present at launch if that goes ahead this October making this kind of check an important safeguard.

The form of this limit may need to be different for different APIs but could include max cost per chunk (or register), and max total cost per call, and preferably both.