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.

dbcon/mysql/ha_mcs_sysvars.h

@@ -30,7 +30,8 @@ extern char cs_commit_hash[];
 // compression_type
 enum mcs_compression_type_t {
     NO_COMPRESSION = 0,
-    SNAPPY = 2
+    SNAPPY = 2,
+    LZ4 = 3
 };
 
 // use_import_for_batchinsert mode