VARIABLES DICREASES UNEXPECTEDLY

Analysis
--------
"SHOW STATUS VARIABLES" shows incorrect status values when
concurrent connection is executing statement to change user
or is being disconnected.

During switch user/session disconnect session status
variables are merged to global status variables. After
this, session variables are not reset for a while. During
this time select of global status variable from another
session will result in wrong data as all the session values
get added to global to calculate final value. Effectively
current session status variable value gets added twice.

Fix:
---
New flag introduced to represent whether THD::status_var
addedd to global status. If status_var_aggregated is set,
THD::status_var is skiped from aggregation.
Code added to reset session variable after adding to global
status variable for change user scenario.

Test:
----
mtr test added for switch user flow. Session disconnect
is tested with debugger as DEBUG_SYNC suite will not
work during thread disconnect.