This commit adds a new build tag `integration` and removes the old tag
`rpctest` for clarity. Multiple unnecessary usages of `build !rpctest`
is also removed.
With go 1.17 a change to the build flags was implemented:
https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md
The formatter now automatically adds the forward-compatible build tag
format and the linter checks for them, so we need to include them in our
code.