MXS-1776: Handle recursive COM_STMT_EXECUTE commands

Readwritesplit would not handle multiple overlapping COM_STMT_EXECUTE
commands properly if they opened cursors. This was due to the fact that
the result would not be marked as complete and COM_STMT_FETCH commands
were executed as if they did not return results.

The correct implementation is to consider a COM_STMT_EXECUTE that opens a
cursor complete only when the first EOF packet is read (that is, when the
resultset header is read). This allows subsequent COM_STMT_FETCH commands
to be handled separately.

The separate COM_STMT_FETCH handling must count the number of packets that
are being fetched. This allows correct tracking of the state of a
COM_STMT_FETCH by checking that the number of packets is correct or the
second EOF/ERR packet is read.

server/core/modutil.cc

@@ -1068,6 +1068,21 @@ int modutil_count_statements(GWBUF* buffer)
     return num;
 }
 
+int modutil_count_packets(GWBUF* buffer)
+{
+    int packets = 0;
+    size_t offset = 0;
+    uint8_t len[3];
+
+    while (gwbuf_copy_data(buffer, offset, 3, len) == 3)
+    {
+        ++packets;
+        offset += gw_mysql_get_byte3(len) + MYSQL_HEADER_LEN;
+    }
+
+    return packets;
+}
+
 /**
  * Initialize the PCRE2 patterns used when converting MySQL wildcards to PCRE syntax.
  */