A watched page generates notifications for events that happened BEFORE user decided to watch it

Steps to reproduce:

• With USER_A:
  • Enable notifications
  • Unwatch the current wiki
• With USER_B
  • Add a comment on Main.WebHome
• With USER_A:
  • Watch the page Main.WebHome
  • Refresh

Expected:

• No notifications concerning Main.WebHome

Actual:

• there is notification for the comment on Main.WebHome, meanwhile the comment has happened BEFORE USER_A decided to watch it.

This is happening since the beginning of the notifications filters, but it requires to revert XWIKI-15440