Fixed Olm session wedging with key forwarding

Changes:

• Extracted m.dummy request to function
• Reverted changes in try_to_forward_room_key and created a separate function forward_room_history_key to perform /keys/claim and forwarding upon Olm session creation
• Updated logic in share_room_history_keys:
  • Moved where session history visibility filtering occurs
  • Changed key forwarding to first send an m.dummy request, followed by /keys/claim request so that forwarded keys are always done in a new Olm session (using existing session could potentially result in a wedged session and losing keys)