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.cpp

@@ -21,8 +21,10 @@
 #include "ha_mcs_sysvars.h"
 
 const char* mcs_compression_type_names[] = {
-    "SNAPPY",
-    "SNAPPY",
+    "SNAPPY", // 0
+    "SNAPPY", // 1
+    "SNAPPY", // 2
+    "LZ4",    // 3
     NullS
 };
 
@@ -39,7 +41,8 @@ static MYSQL_THDVAR_ENUM(
     PLUGIN_VAR_RQCMDARG,
     "Controls compression algorithm for create tables. Possible values are: "
     "NO_COMPRESSION segment files aren't compressed; "
-    "SNAPPY segment files are Snappy compressed (default);",
+    "SNAPPY segment files are Snappy compressed (default);"
+    "LZ4 segment files are LZ4 compressed;",
     NULL, // check
     NULL, // update
     1, //default