This commit turns off route blinding for the daemon while we're waiting
on full handling for blinded errors. The feature remains on for itests
so that tests covering blinding can run as usual.
Add an option to disable route blinding, failing back any HTLC with
a blinding point set when we haven't got the feature enabled.
Note that this commit only handles the case where we're chosen as the
relaying node (where the blinding point is in update_add_htlc), we'll
add handling for the introduction node case once we get to handling of
blinded payloads).
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.
