Updates to specific replication system variables need to target the
active primary connection to support multi-source replication. These
variables use the Sys_var_multi_source_ulonglong type. This class
uses offsets of the Master_info C++ class to generalize access to
its member variables.

The problem is that the Master_info class is not of standard layout,
and neither are many of its member variables, e.g. rli and
rli->relay_log. Because the class is not of standard layout, using
offsets to access member variables invokes undefined behavior.

This patch changes how Sys_var_multi_source_ulonglong accesses the
member variables of Master_info from using parameterized memory
offsets to “getter” function pointers.

Note that the size parameter and assertion are removed, as they are
no longer needed because the condition is guaranteed by compiler
type-safety checks.

Reviewed By:
============
Kristian Nielsen <knielsen@knielsen-hq.org>