MXS-1734 Add possibility to instruct when to dump statements

With the configuration entry

dump_last_statements=[never|on_close|on_error]

you can now specify when and if to dump the last statements
of of a session.

include/maxscale/config.h

@@ -99,6 +99,7 @@ extern const char CN_AUTH_READ_TIMEOUT[];
 extern const char CN_AUTH_WRITE_TIMEOUT[];
 extern const char CN_AUTO[];
 extern const char CN_CONNECTION_TIMEOUT[];
+extern const char CN_DUMP_LAST_STATEMENTS[];
 extern const char CN_DATA[];
 extern const char CN_DEFAULT[];
 extern const char CN_DESCRIPTION[];