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

server/sql_schema/V1_init.sql

@@ -8,7 +8,7 @@ CREATE TABLE features (
   id integer PRIMARY KEY AUTOINCREMENT,
   owner_id integer NOT NULL REFERENCES users(id) ON DELETE CASCADE,
   name text NOT NULL,
-  deadline text,
+  deadline integer,
   properties text NOT NULL,
   geom text NOT NULL
 );

server/sql_schema/V4_user_info.sql

@@ -0,0 +1,4 @@
+ALTER TABLE users ADD COLUMN app_version text;
+ALTER TABLE users ADD COLUMN last_seen_time integer;
+ALTER TABLE users ADD COLUMN last_upload_time integer;
+ALTER TABLE users ADD COLUMN last_download_time integer;