Fix #352 message does not save at session when client is disconnect.

[werbenhu]

For #352, According to MQTT spec 3.1.2.4 Clean Session , if the client does not expire, we should retain the client's subscriptions upon reconnection. However, I tested Mosquitto, and it does not support this feature. Therefore, I think making this an optional capability is a good idea. You can enable this capability by coding as follows:

capabilities := mqtt.DefaultServerCapabilities
capabilities.Compatibilities.TakeoverSubInheritable = true

server := mqtt.New(&mqtt.Options{
    InlineClient: true,
    Capabilities: capabilities,
})

[coveralls]

Pull Request Test Coverage Report for Build 7337508374

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.75%

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Totals
Change from base Build 7294752060:	0.0%
Covered Lines:	5532
Relevant Lines:	5602

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💛 - Coveralls