Draft: Introduce feature INSTANT_FAILOVER_TARGET

See https://github.com/MariaDB/server/pull/3224 and https://github.com/mariadb-corporation/mariadb-connector-c/pull/245

This is still a draft as unfortunately the feature does not seem to fully work. The configuration works correctly on the server side, e.g.:

 ../client/mariadb --defaults-file=/tmp/build/source/builddir/mysql-test/var/log/main.instant_failover/my.cnf --host 127.0.0.1 -e 'STATUS; SHOW GLOBAL VARIABLES LIKE "instant_failover_%";'
--------------
../client/mariadb from 11.4.2-MariaDB, client 15.2 for debian-linux-gnu (x86_64) using  EditLine wrapper

Connection id:		4
Current database:	
Current user:		root@localhost
SSL:			Cipher in use is TLS_AES_256_GCM_SHA384, cert is UNKNOWN
Current pager:		stdout
Using outfile:		''
Using delimiter:	;
Server:			MariaDB
Server version:		11.4.2-MariaDB-3-log Debian n/a
Protocol version:	10
Connection:		127.0.0.1 via TCP/IP
Server characterset:	latin1
Db     characterset:	latin1
Client characterset:	latin1
Conn.  characterset:	latin1
TCP port:		16000
Uptime:			1 min 6 sec

Threads: 1  Questions: 9  Slow queries: 0  Opens: 17  Open tables: 10  Queries per second avg: 0.136
--------------

+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| instant_failover_mode   | OFF   |
| instant_failover_target |       |
+-------------------------+-------+

../client/mariadb --defaults-file=/tmp/build/source/builddir/mysql-test/var/log/main.instant_failover/my.cnf --host 127.0.0.1 -e 'set global instant_failover_target="127.0.0.2";set global instant_failover_mode=ON;'

However the client side does not seem to follow the redirect, and just fails to connect no matter what I try:

../client/mariadb --defaults-file=/tmp/build/source/builddir/mysql-test/var/log/main.instant_failover/my.cnf --follow-instant-failovers --reconnect --host 127.0.0.1             
ERROR 4196 (HY000): |Server is directing clients to the alternative server '127.0.0.2'|127.0.0.2

This problem is also reflected in the MTR:

main.instant_failover                    [ fail ]
        Test ended at 2024-07-07 00:27:08

CURRENT_TEST: main.instant_failover
ERROR 4196 (HY000): |Server is directing clients to the alternative server '127.0.0.1:16001'|127.0.0.1:16001
mysqltest: At line 31: query 'connect OKAY,127.0.0.1,root,,test,$MASTER_MYPORT' failed: ER_INSTANT_FAILOVER (4196): |Server is directing clients to the alternative server '127.0.0.1:16001'|127.0.0.1:16001

The result from queries just before the failure was:
call mtr.add_suppression("\\[Warning\\] Redirecting connection \\d+ via \\S+ to INSTANT_FAILOVER_TARGET=\\S+ \\(INSTANT_FAILOVER_MODE=\\w+\\)");
Verify that a TCP connection works with --disable-follow-instant-failovers (before enabling instant failover)
default_port_tcp_works
1
Verify that connections via local socket and extra port SUCCEED
connection default;
connect local_sock,localhost,root,,test;
disconnect local_sock;
connect tcp_sock,127.0.0.1,root,,test,$MASTER_EXTRA_PORT;
disconnect tcp_sock;
Enable instant failover in its default mode (ON)
connection default;
set global instant_failover_mode=ON;
set global instant_failover_target="127.0.0.1:$MASTER_EXTRA_PORT"
With --disable-follow-instant-failovers, connecting to the default port should now fail
With --follow-instant-failovers (the client library default), this should redirect to the extra port