Better handling of deposit for register

[PiVortex]

1. In shade-agent-js there is a fixed deposit. If one changes the contract so that it requires more storage deposit which will likely happen then shade-agent-js can't handle this nicely. Add an optional parameter to register to set the deposit amount.

2. If an agent re-registers whilst its already registered then it is required to deposit again. It shouldnt need to as no more storage is being used up. Check if its already registered and if so don't require deposit. register in shade-agent-js can check if registered before hand and attach deposit accordingly, but also have an optional argument to force deposit or force no deposit (no check done)

3. A contract owner can keep changing measurements or removing agents, or if an agent tries to peform an action before reregistering and its attestation has expired. This will cause them to have to repay storage. Have a think if there is a nice way to handle this, if not its not a huge issue.