Revise Utils::Gzip::compress code

Change signature
Add ZLIB_CONST define to make  z_stream.next_in const
Cast to zlib defined type Bytef*
Set memLevel to 9 in deflateInit2() for maximum performance
Revise compression loop
On returning false, free memory correctly by calling deflateEnd()
Reserve space by the estimation of deflateBound()

src/base/utils/gzip.h

@@ -36,7 +36,7 @@ namespace Utils
 {
     namespace Gzip
     {
-        bool compress(QByteArray src, QByteArray &dest);
+        QByteArray compress(const QByteArray &data, int level = 6, bool *ok = nullptr);
         bool uncompress(QByteArray src, QByteArray &dest);
     }
 }