MCOL-987 Add LZ4 compression.

* Adds CompressInterfaceLZ4 which uses LZ4 API for compress/uncompress.
* Adds CMake machinery to search LZ4 on running host.
* All methods which use static data and do not modify any internal data - become `static`,
  so we can use them without creation of the specific object. This is possible, because
  the header specification has not been modified. We still use 2 sections in header, first
  one with file meta data, the second one with pointers for compressed chunks.
* Methods `compress`, `uncompress`, `maxCompressedSize`, `getUncompressedSize` - become
  pure virtual, so we can override them for the other compression algos.
* Adds method `getChunkMagicNumber`, so we can verify chunk magic number
  for each compression algo.
* Renames "s/IDBCompressInterface/CompressInterface/g" according to requirement.

writeengine/bulk/we_colbufcompressed.h

@@ -107,8 +107,7 @@ private:
     // should always be 4MB, unless
     // working with abbrev extent.
     size_t               fNumBytes;             // num Bytes in comp buffer
-    compress::IDBCompressInterface*
-    fCompressor;           // data compression object
+    compress::CompressorPool fCompressorPool;   // data compression object pool
     compress::CompChunkPtrList
     fChunkPtrs;            // col file header information
     bool                 fPreLoadHWMChunk;      // preload 1st HWM chunk only