[lib] move generating P2P messages to useProcessDMOperation

Summary:
ENG-8423.

We need to process DM Operations and save OutboundP2PMessages in DB in the same SQL transaction to handle cases when the app crashes after applying changes to our store and before addressing messages to other peers - this is inconsistency from which we're unable to heal.

The existing place of generating messages is removed later in the stack.

Depends on D13148

Test Plan:
Check that messages are persisted along with changes to store.

Note: This stack is tested end-to-end according to the process described here.

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13149