Collecting usage info, change sqlite lib.

* X-User-ID header is processed to get user ID.
* Time of last request for a user is saved into DB.
* Time of last upload and download is stored for a user.
* Added DB migration to add columns into users table to store the times and app version.
* Backward fix of datatype of the deadline column in features table.
* Switched from crawshaw.io/sqlite to zombiezen.com/go/sqlite.
  * Refactored DB handling.
  * Used migration routine from zombiezen in favour of manual one.
  * Runtime DB reinit simply deletes the db file and initializes the db anew.

Fix #6

models/models.go

@@ -19,6 +19,14 @@ type User struct {
 	Name string `json:"name"`
 }
 
+type UserInfo struct {
+	User
+	AppVersion       *semver.Version `json:"app_version"`
+	LastSeenTime     *time.Time      `json:"last_seen_time"`
+	LastUploadTime   *time.Time      `json:"last_upload_time"`
+	LastDownloadTime *time.Time      `json:"last_download_time"`
+}
+
 type Feature struct {
 	// ID is an ID of the feature.
 	// When the feature is submitted by a client for creation (i.e. in Update.Create) it is considered a 'local' ID which must be unique across all submitted features.