Messaging - Cache messages for getVal

[mkrebs81]

Issue

Messaging-based devices have issues using getVal() in certain situations:

• data points with no active read option (no outgoing read command, only passive incoming message)
• data point has not yet received a value from the device
• subscribe() was not called in commhandler (actual messaging client will not subscribe to anything otherwise!)

Example: Intermediary has difficulties in situation described above, as reported by David Zogg.

Potential Solution

Improve the existing message cache.

• use cache not only when message is received through subscribe method, but always
• allow getVal read from cache, if value is not too old
• introduce per-device cache lifetime, since this is not yet available in SGr specification
• may need to manage topic subscriptions internally to make it work without external subscribe() call