Migrate @@rpl_semi_sync_slave_enabled to per-connection

In multi-master one can have semi-sync enabled for some connection and not for others by changing rpl_semi_sync_slave_enabled when connecting to the master.
In the future we should change things so that semi-sync should be a parameter for CHANGE MASTER so that one can have different semi-sync options for each connection.
⸺ monty's rejection on MDEV-32944

It is unïntuitive that @@rpl_semi_sync_slave_enabled can be changed mid-replication, but only takes effect on new connection establishments.

MDEV-32947 reported that @@rpl_semi_sync_slave_enabled beïng global is a bug because it can change mid-replication.
It'd also be unïntuitive if changes to this variable only take effect after every IO thread stops, so it should be a per-connection configuration in practice.

A new configuration is required, for directly changing the variable to the @@connection.variable syntax would break whatever compatibility it's still worth.
(It becomes specific to @@default_master_connection when connection is not specified.)
Though CHANGE MASTER rpl_semi_sync_slave_enabled, albeit verbose, would match this variable with MDEV-28302's conventions.

@@rpl_semi_sync_slave_delay_master and @@rpl_semi_sync_slave_trace_level also make sense to migrate to per-connection, though this process can also expand their effects to non-semi-sync as well.
MDEV-39583 will make @@rpl_semi_sync_slave_kill_conn_timeout obsolete.