[lib] update P2P message handler to use sequential decrypt

Summary:
In client <-> client communication we have to track messages order.

Depends on D11719

Test Plan:
Created session between two clients:

1. Sending a couple of encrypted messages
2. Sending messages with wrong order -> OLM_MESSAGE_OUT_OF_ORDER
3. Sending the same message multiple times -> OLM_ALREADY_DECRYPTED_OR_KEYS_SKIPPED

Reviewers: tomek, marcin

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11720