[Bugfix] FIFO buffers even when network up

Values are buffered then sent, even when network is up. This fixes
issues in case of network failure: buffered value sets were sent
"interleaved" with real time data, messing up with post-processing.

Issue discussed here:
https://github.com/Jerome-github/oem_gateway/issues/6

OemGatewayBuffer:
- remove send() method
- add add() method to buffer data
- remove unused run() method
- add flush() method to send data
- _send_data(): time parameter no more optionnal