Remove unnecessary bootstrap calls

[birtony]

Once trustbloc/auth#282 and trustbloc/agent-sdk#392 are completed, we should update our logic to avoid making unnecessary calls to bootstrap endpoint. Wallet should only call it when requestAccess() or continue() return a corresponding flag that indicates that the user had already been registered in the system before. In this case, we'd need to fetch bootstrap data to pass it to agent opts. Once agent is initialized and posted to bootstrap successfully, it would return bootstrap data to wallet directly with the implemented JS binding.