Bugfix/dont send ack on new connection

[SgtSilvio]

Motivation
MQTT 5 specification

When a Client reconnects with Clean Start set to 0 and a session is present, both the Client and Server MUST resend any unacknowledged PUBLISH packets (where QoS > 0) and PUBREL packets using their original Packet Identifiers. This is the only circumstance where a Client or Server is REQUIRED to resend messages. Clients and Servers MUST NOT resend messages at any other time [MQTT-4.4.0-1].

Currently it is possible that PUBACK and PUBREC packets can be sent on a new connection after a reconnect.
Can be reproduced by:

1. using manual acknowledgement
2. receive a message
3. reconnect the client
4. acknowledge the message before the server resends the corresponding PUBLISH message

Changes

• Added a connectionIndex to pending incoming messages