MINOR: compression: memlevel and windowsize

The window size and the memlevel of the zlib are now configurable using
global options tune.zlib.memlevel and tune.zlib.windowsize.

It affects the memory consumption of the zlib.
diff --git a/include/types/global.h b/include/types/global.h
index 28632b7..2f41be5 100644
--- a/include/types/global.h
+++ b/include/types/global.h
@@ -112,6 +112,10 @@
 #ifdef USE_OPENSSL
 		int sslcachesize;  /* SSL cache size in session, defaults to 20000 */
 #endif
+#ifdef USE_ZLIB
+		int zlibmemlevel;    /* zlib memlevel */
+		int zlibwindowsize;  /* zlib window size */
+#endif
 	} tune;
 	struct {
 		char *prefix;           /* path prefix of unix bind socket */