* This patch extends CompressedDBFileHeader struct with new fields:
`fColumWidth`, `fColDataType`, which are necessary to rebuild extent map
from the given file. Note: new fields do not change the memory
layout of the struct, because the size is calculated as
max(sizeof(CompressedDBFileHeader), HDR_BUF_LEN)).
* This patch changes API of some functions, by adding new function
argument `colDataType` when needed, to be able to call `initHdr`
function with colDataType value.
Add more comments.
Changed return value for HDFS'es fallocate.
Removed unnecessary code in ColumnBufferCompressed::writeToFile
Replaced Nulls with Empties in variable names.
