MXS-1503: Enable master reconnection

The `master_reconnection` parameter now controls both the reconnection of
the master server as well as the migration of the master server to another
server. Although these two cases appear to be different, the end result
from readwritesplit's point of view is the same and are thus controlled
with the same parameter.

The RWBackend class now resets its internal state when it is closed. This
allows readwritesplit to handle the case when a result was expected from
the master but the master died before the result was returned. The same
code should also handle slave connection failures mid-result, allowing
Backend reuse.

Added a test case that verifies the new functionality when combined with
`master_failure_mode=error_on_write`.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -66,6 +66,7 @@ public:
 
     bool execute_session_command();
     bool write(GWBUF* buffer, response_type type = EXPECT_RESPONSE);
+    void close(close_type type = CLOSE_NORMAL);
 
     inline void set_large_packet(bool value)
     {