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/shared/we_bulkrollbackfilecompressed.h

@@ -28,6 +28,7 @@
 
 #include <cstdio>
 #include <cstring>
+#include <unordered_map>
 
 #include "we_define.h"
 #include "we_type.h"
@@ -148,7 +149,7 @@ private:
                                 uint64_t&   ptrHdrSize,
                                 std::string& errMsg ) const;
 
-    compress::IDBCompressInterface fCompressor;
+    compress::CompressorPool fCompressorPool;
 };
 
 } //end of namespace