bug #26842: master binary log contains invalid queries - replication fails
bug #12826: Possible to get inconsistent slave using SQL syntax Prepared Statements

Problem:  
binlogging PS' we may produce syntacticly incorrect queries in the binlog replacing 
some parameters with variable names (instead of variable values).
E.g. in the reported case of "limit ?" clause: replacing "?" with "@var"
produces "limit @var" which is not a correct SQL syntax. 
Also it may lead to different query execution on slave if we
set and use a variable in the same statement, e.g.
"insert into t1 values (@x:=@x+1, ?)"

Fix: make the stored statement string created upon its execution use variable values
(instead of names) to fill placeholders.