WL#5791: Add user variables as parameters in Binlog::log_statement

This is a step towards making replication a library, such that the 5.6 rpl-lib can 
be plugged into the 5.5 core [WL#5675].

In particular, this is a step toward removing function arguments of type THD* from 
the interface. This is needed because THD is not stable.

In this worklog, we make the class function Binlog::log_statement() take a list of 
user variables as one parameter. Before this worklog, user variables were read 
from the THD argument.

See also WL#5790.