Due to the old heartbeat of the high latency telemetry when activating it after flying sufficiently long in normal telemetry it is first detected as lost until the first message is sent.
By updating the heartbeat to the current time on switching this issue is avoided.

Also includes a small style fix for the HIGH_LATENCY2 stream

- Check if changing to a non standby state if the current state is the standy state and no change is already scheduled.
- Add prefix _ to all class variables

- Allow using the full buffer size (340 bytes)
- Add tx mutex for sbd session to avoid dropping message which are written during an sbd session

- Every time data is written to the iridium module all previous untransmitted data is overwritten
- The buffer is reset only on the following conditions
    - A sbd session is successfully completed
    - An incoming mavlink message would result in less than SATCOM_MIN_TX_BUF_SPACE free bytes after it is written
    - Any other incomming data would result in less than SATCOM_MIN_TX_BUF_SPACE free bytes after it is written to the buffer.

- Do not send any mission updates as they are handled within HIGH_LATENCY2
- Always send a command acknowledge even if transmitting is not enabled

- Reorder functions so that they have the same order in the .h and .cpp file
- Update documentation for functions
- Make variables and some functions private

Switch to the high latency telemetry, if available, if every low latency link is lost. Switch back if any low latency link is regained. Only indicate that all links are lost if all high and low latency links are lost. Allow different timeouts for high and low latency links.

- Update heartbeat after successful sbd session and fill it in the telemetry_status message
- Add parameter for session timeout, read interval, and stacking time
- Add sbd session timeout
- Fix updating the rssi at a constant rate when no sbd session is scheduled
- Add variable timout to read_at function
- Check if test command is valid to avoid being stuck in the test state