OemGatewayBuffer: don't flush buffer if inactive
OemGatewayBuffer: sending period

Send data to server once every N seconds
"period" parameter is added, config file adaptation required

This commit sort of reverts 2d96eea700db63b600baceb5a3d77d7d6b966573
OemGatewayEmoncmsBuffer: MAX_DATA_SETS_PER_POST

Do not send more than 100 data sets in one post when flushing the buffer.
The aim is to avoid long URL after a long network downtime.