Fixed write rejection when wal size exceeds threshold

[caozj1011]

When the wal file disk size exceeds 50G x 0.8, the flush or snapshot logic will be triggered. However, when the active memtable is large enough, the snapshot or flush speed will always be lower than the new generation speed, so the write is always refused. The fix determines that if the threshold is exceeded, the snapshot or flush operation continues until the disk size is below the threshold. In addition, also changed the judgment conditions, the return from trySnapshotOrFlushMemTable return after failure to success