WIP: The Great Configuration Variables Exodus (deprecation)

[shlomi-noach]

This (ongoing) PR deprecates configuration variables that are:

• unused
• not so important to have configurable (can do with magic values)
• used to be used for backwards compatibility but should now just go away
• can be superseded by other solutions

[shlomi-noach]

Listing of deprecated variables (to be edited as PR proceeds)

• DatabaselessMode__experimental -- with ongoing work to run on sqlite this variable becomes irrelevant.
• BufferBinlogEvents -- should always be true. It works better and places less locks on the backend server.
• BinlogFileHistoryDays is now just a magic number.
• MaintenanceOwner is now a magic string.
• ReadLongRunningQueries -- it's just not orchestrators job! Removing this feature.
• DiscoveryPollSeconds was already deprecated. Now a magic number.
• ActiveNodeExpireSeconds is now a magic number. Since DiscoveryPollSeconds is 1sec, ActiveNodeExpireSeconds can follow suit.
• AuditPageSize is now a magic number
• AuditPurgeDays is now a magic number (31)
• SlaveStartPostWaitMilliseconds now deprecated. Automated, incremental sleep tries to minimize sleep time.
• MySQLTopologyMaxPoolConnections is now a constant
• MaintenancePurgeDays is now a constant
• MaintenanceExpireMinutes is now a constant
• HttpTimeoutSeconds changed to AgentHttpTimeoutSeconds and is now a constant
• AgentAutoDiscover is deprecated. Assumed true
• PseudoGTIDCoordinatesHistoryHeuristicMinutes is now a constant

[shlomi-noach]

The list is growing and the PR is open for too long. I will resolve + merge this PR and will open another one for more deprecation.