fix issue #766

[pprasse]

This Pull-RQ fixes #766
The SESSION_TRACK_SYSTEM_VARIABLES field type was not backported, which led to incorrect handling when the system variable "character_set_client" changed (as was the case in #766 )

I also backported SESSION_TRACK_TRANSACTION_STATE, SESSION_TRACK_TRANSACTION_CHARACTERISTICS, SESSION_TRACK_SCHEMA as we should also handle those in accordance with the standard mysql client library.

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[lth]

I think a simpler fix is to remove the types from the switch statement and have it fall back to the default case which will handle unknown types correctly by skipping over them: https://github.com/Actindo/mysql-5.6/blob/bc6a6f44b3d4fabfa937d756057af996f3dbb1d6/sql-common/client.c#L893

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