Address two bugs found by code review:
1. MemoryCryptoStore loses all E2EE keys on restart — now pickle the
store to disk on disconnect and restore on connect, preserving
Megolm sessions across restarts.
2. Encrypted events buffered for retry were silently dropped after
decryption because _on_encrypted_event registered the event ID
in the dedup set, then _on_room_message rejected it as a
duplicate. Now clear the dedup entry before routing decrypted
events.
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