Refactor ftable schema

[andyfengHKU]

• Split FTableSchema, FactorizedTableUtils into their own files.
• Move dataChunkPos out of ColumnSchema. The previous design simply doesn't make sense. We now maintain this information at FTableSchema. The high level idea is that FTableSchema contains multiple ColumnSchema and also multiple Group. Each ColumnSchema belongs to one Group. If multiple ColumnSchema belongs to the same Group then they must share the same Group metadata, i.e. flat information. There is still more work to do, for example we should make GroupID consecutive instead of using DataChunkPos. But this needs to be done progressively.
• Remove unnecessary unique_ptr
• Protect implicit copy with EXPLICIT_COPY macro.