identity: preserve session refresh schedule

The databroker identity manager is responsible for refreshing session
records, to account for overall session expiration as well as OAuth2
access token expiration.

Refresh events are scheduled subject to a coolOffDuration (10 seconds,
by default) relative to a lastRefresh timestamp. Currently, any update
to a session record will reset the associated lastRefresh value and
reschedule any pending refresh event for that session. If an update
occurs close before a scheduled refresh event, this will push back the
scheduled refresh event to 10 seconds from that time.

This means that if a session is updated frequently enough (e.g. if there
is a steady stream of requests that cause constant updates via the
AccessTracker), the access token may expire before a refresh ever runs.

To avoid this problem, do not update the lastRefresh time upon every
session record update, but only if it hasn't yet been set. Instead,
update the lastRefresh during the refresh attempt itself.

Add unit tests to exercise these changes. There is a now() function as
part of the manager configuration (to allow unit tests to set a fake
time); update the Manager to use this function throughout.

internal/identity/manager/data.go

@@ -54,6 +54,9 @@ func (u *User) UnmarshalJSON(data []byte) error {
 // A Session is a session managed by the Manager.
 type Session struct {
 	*session.Session
+	// lastRefresh is the time of the last refresh attempt (which may or may
+	// not have succeeded), or else the time the Manager first became aware of
+	// the session (if it has not yet attempted to refresh this session).
 	lastRefresh time.Time
 	// gracePeriod is the amount of time before expiration to attempt a refresh.
 	gracePeriod time.Duration