Resolve "FEATURE: Use time stamp from kurisu in update process"

Closes #109 (closed)

• New way of updating the db by re-using fileds when possible
• Check with the repo_timestamp in the repo module instead of getting the file's mtime
• Default the repo_timestamp to the file's mtime at upgrade time

inc/lektor/internal/dbversion.def

@@ -4,5 +4,6 @@ database_version("alpha",  ALPHA,  0)
 /* Next versions of the database! */
 database_version("mk-7.1", MK_7_1, 1)
 database_version("mk-7.2", MK_7_2, 2)
+database_version("mk-7.3", MK_7_3, 3)
 
 // vi:syntax=c