Duplicate PageFile make TiFlash can not GC delta data

[JaySon-Huang]

We found that there is a chance that page_3808_1 and legacy.page_3808_1 exist at the same time and make trouble for GCing DeltaTree's delta data.

248K	data/t_131/log/legacy.page_3808_1
378M	data/t_131/log/page_3808_1

It will make the GC thread throw "Directory not empty" like this

2021.06.11 12:48:19.919511 [ 5 ] <Error> void DB::BackgroundProcessingPool::threadFunction(): Poco::Exception. Code: 1000, e.code() = 39, e.displayText() = Directory not empty: /data1/tidb-data/tiflash-9000/data/t_131/log/page_3808_1, e.what() = Directory not empty