Add feature flagged support for issuing for IDNs, fixes#597.
This patch expects that clients have performed valid IDN2008 encoding on any label that includes unicode characters. Invalid encodings (including non-compatible IDN2003 encoding) will be rejected. No script-mixing or script exclusion checks are performed as we assume that if a name is resolvable that it conforms to the registrar's policies on these matters and if it uses non-standard scripts in sub-domains etc that browsers should be the ones choosing how to display those names.
Required a full update of the golang.org/x/net tree to pull in golang.org/x/net/idna, all test suites pass.
This PR adds a migration to create two new fields on the `certificateStatus` table: `notAfter` and `isExpired`. The rationale for these fields is explained in #1864. Usage of these fields is gated behind `features.CertStatusOptimizationsMigrated` per [CONTRIBUTING.md](https://github.com/letsencrypt/boulder/blob/master/CONTRIBUTING.md#gating-migrations). This flag should be set to true **only** when the `20160817143417_CertStatusOptimizations.sql` migration has been applied.
Points of difference from #2132 (the initial preparatory "all-in-one go" PR):
**Note 1**: Updating the `isExpired` field in the OCSP updater can not be done yet, the `notAfter` field needs to be fully populated first - otherwise a separate query or a messy `JOIN` would have to be used to determine if a certStatus `isExpired` by using the `certificates` table's `expires` field.
**Note 2**: Similarly we can't remove the `JOIN` on `certificates` from the `findStaleOCSPResponse` query yet until all DB rows have `notAfter` populated. This will happen in a separate **Part Two** PR.
Updates #1699.
Adds a new package, `features`, which exposes methods to set and check if various internal features are enabled. The implementation uses global state to store the features so that services embedded in another service do not each require their own features map in order to check if something is enabled.
Requires a `boulder-tools` image update to include `golang.org/x/tools/cmd/stringer`.
Roland Bracewell Shoemaker