Buffer: store data when network is down

[lafrech]

Currently, when the buffer can't reach its target, the data is kept in RAM and the communication retried later. To avoid filling up the RAM, the data is trashed after an arbitrarily hardcoded amount of data.

Possible improvements:

• Let the user parametrize the buffer size (not that useful)
• Optionally write data in a file from times to times (not too often to avoid SD card wearoff, but enough to avoid data loss on power shortage)