Priority based flush handler execution

[xichen2020]

cc @martin-mao

The flush handlers for metrics with higher resolutions should be given higher priority compared to those with lower resolutions to ensure alerts and dashboards are still working in cases where the agg servers are temporarily overloaded. This could be done by assigning different priorities to items queued on the bucket heap where buckets with high resolutions have higher priorities and are at the top of the heap and as such will be flushed earlier than those with lower resolutions.