Change default Root compression to LZMA:4

LHCb measured that LZMA:4 is considerably faster than
LZMA:6, with only a small increase in size.
See LBCORE-1116.

See merge request !239

RootCnv/src/RootDataConnection.cpp

@@ -25,7 +25,7 @@
 #include "TBranch.h"
 #if ROOT_VERSION_CODE >= ROOT_VERSION(5,33,0)
 #include "Compression.h"
-static int s_compressionLevel = ROOT::CompressionSettings(ROOT::kLZMA,6);
+static int s_compressionLevel = ROOT::CompressionSettings(ROOT::kLZMA,4);
 #else
 static int s_compressionLevel = 1;
 #endif