Update Svc/TlmChan to send multiple channels per ComBuffer

[timcanham]

F´ Version	3.0
Affected Component	Svc/TlmChan

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Feature Description

Put more than one channel in ComBuffer when downlinking updated channels. Fill the ComBuffer with channels until it is full and repeat until all updated channels are sent.

Rationale

Svc/TlmChan is currently very inefficient in that it sends only one channel per downlink packet. This will make it more efficient per packet.

Note: A ground system update is needed for this since the GDS only extracts one telemetry channel from each packet.

nasa/fprime-gds#40

[LeStarch]

Done #1523