fix: set error to ERR_SESSION_RESET to trigger meta query while se…

[neverchanje]

…ssion not responding.

Refer to #53

How does it work:

-            closeSession();
+            closeSession(); // maybe fail when the session is already disconnected.
+           errno = error_types.ERR_SESSION_RESET;

After close the session, we set error to ERR_SESSION_RESET, this error will definitely trigger
meta query, to update the local configuration table.

A potential problem is that every 10 seconds of continuous timeout errors will incur one meta query.
It may cause overload on meta-server. For example, when there are 100 clients concurrently loading data to Pegasus which causes many timeouts, the 100 clients will altogether request the single meta-server every 10 seconds.

In most cases, the scalability won't be a problem, but it should be awared.