WIKI-794 Failed to access huge wiki pages after version history migration

Problem analysis
* WIKI-746 runs version migration when an old page is opened the first time.
  The deletion of old page history takes time and cannot be finished due to transaction timeout.

Fix description
* During version migration, page history deletion is moved to a dedicated thread to avoid the transaction timeout.
  When timeout happens, these old data are not removed but it doesn't block the page to render correctly.