Redesign the single file format for DTFile

We propose a "single file mode" for DTFile to store the snapshot generated / snapshot files for ingesting. Storing data in a single file is easy for transferring data between nodes.
Single File Format

But there are some problems for current design and implementation:

1. The file format is not efficient. For example, we need to open and read the same file four times to get Footer, meta, pack_stat, and sub_file_stat (all of them are metadata of a DTFile). The system calls and small reads hurt performance.
   https://github.com/pingcap/tics/blob/c72c331726b45197f45a8d2af766070b97b2b25f/dbms/src/Storages/DeltaMerge/File/DMFile.cpp#L262-L334
2. There are a lots of isSingleFileMode() / if (mode == Mode::FOLDER) inside DTFile implementation, which make it messy. We can split the logic into two subclasses DTFileDirectory, DTFileSingle, and make the codes more clear.