* Use features from Go 1.16 tooling.
- GO111MODULE environment variable now defaults to "on"
- "go build" and "go test" now exit with an error rather than silently modifying go.mod or go.sum files
- Update README.md
* Don't use deprecated ioutil package.
* Remove unnecessary error handling.
* Generate fee addresses in dcrvsp, not dcrwallet
* Break loop if multiple invalid children are generated.
* Use Mutex instead of RWMutex
* Rework client/server authentication.
- Remove Signature from all requests, and instead expect a signature in HTTP header "VSP-Client-Signature".
- Remove CommitmentSignatures from the database.
- Use a bool flag to indicate when a ticket is missing from the database rather than an error.
This commit introduces a lot of duplication into each of the authenticated HTTP handlers. This should be removed in future work which moves the authentication to a dedicated middleware.
* Introduce auth and rpc middleware.
This removed the duplication added in the previous commit, and also removes the duplication of RPC client error handling.
