Reduce the memory cost when there are stale snapshots for PageStorage

[JaySon-Huang]

We meet some extreme cases that block PageStorage GC from running normally, for example:

• Moderate GC policy leaves lots of legacy PageFile TiFlash OOM because of too many legacy PageFile are not GC #1550
• Duplicated PageFile throw exception in GC Duplicate PageFile make TiFlash can not GC delta data #2169
• MPP tasks may hold streams (and related segment snapshot) after canceled MPPTask::runImpl is not exception safe #2322

These extreme cases/bugs leave lots of PageFile on disks. When running GC, we open lots of (several hundred to thousands of) PageFiles and read all theirs meta parts from data in PageFile::MetaMergingReader::initialize.

https://github.com/pingcap/tics/blob/ec5f976a8fb85db497d3f9f67cd0717885d8075a/dbms/src/Storages/Page/gc/LegacyCompactor.cpp#L110-L119
https://github.com/pingcap/tics/blob/ec5f976a8fb85db497d3f9f67cd0717885d8075a/dbms/src/Storages/Page/PageFile.cpp#L249-L255

Assume that each meta part of one PageFile is 500KiB, if there are 2000 PageFiles left on disk, then each round of GC we need to scan 1 GiB.
Instead of reading all meta parts of (thousands of) PageFiles once, we can allocate a smaller buffer size in PageFile::MetaMergingReader::initialize, and read the rest data from the disk while running PageFile::MetaMergingReader::moveNext.
This change will keep some file descriptor opened for a while and call ::read several times, but it can reduce the memory cost when there are lots of PageFiles.

[JaySon-Huang]

Instead of reading all meta parts of (thousands of) PageFiles once, we can allocate a smaller buffer size in PageFile::MetaMergingReader::initialize, and read the rest data from the disk while running PageFile::MetaMergingReader::moveNext.
This change will keep some file descriptor opened for a while and call ::read several times, but it can reduce the memory cost when there are lots of PageFiles.

• Related intro doc for PageStorage (not up-to-date): DeltaMerge Storage Engine in Alpha
• Check dbms/src/IO/ReadBufferFromFileDescriptor.h, it should be helpful for this task

@jiangyuzhao

[JaySon-Huang]

An example for if we don't resolve this issue:

[JaySon-Huang]

close as PageStorage V3 is released on v6.2.0 and this problem is addressed in #4989