db_bitcask: Offset calls to bitcask:merge on different nodes
db_bitcask: Offset calls to bitcask:merge on different nodes

Bitcasks merge is expensive, causing large latency spikes when all nodes
merge at the same time. Non-overlapping merges prevent spikes for the
majority of tested configurations.

scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris