Insert unseq file to seq files when possible

We judge if data belongs to the seq region or unseq region only by the lastFlushedTime of each device, which makes the following situation possible:
2 seq files and their time ranges: seq1[0, 99], seq2[300, 500]
1 unseq file and its time range: unseq1[150, 200]
Intuitively, we can just make them 3 seq files, which may potentially reduce query overhead.

One solution may be: right after we close one unseq file, we check if it actually overlaps with any seq file(s), and if not, we insert it into a proper position of the seq files instead of keeping it in unseq files. Another merit is that this avoids the case mentioned in https://github.com/apache/iotdb/pull/2920#pullrequestreview-623932806, because each unseq file will definitely overlap at least one seq file.