MXS-827: Add connection keepalive

The readwritesplit now sends COM_PING queries to backend servers that have
been idle for too long. The option is configured with the
`connection_keepalive` parameter.

include/maxscale/modutil.h

@@ -76,6 +76,14 @@ mxs_pcre2_result_t modutil_mysql_wildcard_match(const char* pattern, const char*
  */
 char* modutil_MySQL_bypass_whitespace(char* sql, size_t len);
 
+/**
+ * @brief Write a COM_PING to a DCB and ignore the response
+ *
+ * @param dcb The backend DCB where the COM_PING is written
+ * @return True if command was successfully sent
+ */
+bool modutil_ignorable_ping(DCB *dcb);
+
 /** Character and token searching functions */
 char* strnchr_esc(char* ptr, char c, int len);
 char* strnchr_esc_mysql(char* ptr, char c, int len);