internal charset to one associated with currently being handled query. 
To note such a query can come from interactive client either.

There was a discussion within replication team and Monty who's suggestion won.
It avoids straightforward parsing of all `set' queries that could affect client side 
character set. 
According to the idea, mysql client does not parse `set' queries but rather cares of
`charset new_cs_name' command.
This command is generated by mysqlbinlog in form of exclaiming comment (Lars' suggestion)
so that enlightened clients like `mysql' knows what to do with it.

Interactive human can switch between many multi-byte charsets during the session 
providing the command explicitly. 
To note that setting new internal mysql's charset does not
trigger sending any `SET' sql statement to the server.