Refactor Master & Relay Log info to iterable tuples

Currently,

• Loose and repeated code provides the loading of the info files as well as te saving of the @@relay_log_info_file.
• One single hard-to-track text formatting drives the saving of the @@master_info_file.

By iterating through collections of polymorphic instances, we not only avoid both issues for both operations, but can also expect the loops to be more performant.

Since all of the saved info is visible in SHOW REPLICA STATUS, the command can also optionally use a polymorphic method for writing its fields.
The same can't be guaranteed for the lexing/parsing for CHANGE MASTER currently, but it would nonetheless help with eventually eliminating repetitions in the code for evaluating.