storage: fix postgres timezone handling

Dex's Postgres client currently uses the `timestamp` datatype for
storing times. This lops of timezones with no conversion, causing
times to lose locality information.

We could convert all times to UTC before storing them, but this is
a backward incompatible change for upgrades, since the new version
of dex would still be reading times from the database with no
locality.

Because of this intrinsic issue that current Postgres users don't
save any timezone data, we chose to treat any existing installation
as corrupted and change the datatype used for times to `timestamptz`.
This is a breaking change, but it seems hard to offer an
alternative that's both correct and backward compatible.

Additionally, an internal flag has been added to SQL flavors,
`supportsTimezones`. This allows us to handle SQLite3, which doesn't
support timezones, while still storing timezones in other flavors.
Flavors that don't support timezones are explicitly converted to
UTC.

storage/storage.go

@@ -44,11 +44,9 @@ type GCResult struct {
 	AuthCodes    int64
 }
 
-// Storage is the storage interface used by the server. Implementations, at minimum
-// require compare-and-swap atomic actions.
-//
-// Implementations are expected to perform their own garbage collection of
-// expired objects (expect keys, which are handled by the server).
+// Storage is the storage interface used by the server. Implementations are
+// required to be able to perform atomic compare-and-swap updates and either
+// support timezones or standardize on UTC.
 type Storage interface {
 	Close() error