IF: Support irreversible read-mode under Savanna

[heifner]

Depends on #2057.

Currently --read-mode irreversible does not work in instant finality mode.

info  2024-03-06T15:06:06.458 nodeos    producer_plugin.cpp:757       on_incoming_block    ] Received block 95af53bc64d04208... #123 @ 2024-03-06T15:06:06.500 signed by eosio [trxs: 1, lib: 122, net: 0, cpu: 0, elapsed: 0, time: 0, latency: -41 ms]
debug 2024-03-06T15:06:06.458 nodeos    net_plugin.cpp:3243           update_chain_info    ] updating chain info lib 122, fork 123
debug 2024-03-06T15:06:06.458 net-0     net_plugin.cpp:3883           operator()           ] accepted signed_block : #123 95af53bc64d04208...
debug 2024-03-06T15:06:06.889 net-2     net_plugin.cpp:3112           process_next_block_m ] ["localhost:9776:blk - b741d54" - 1 127.0.0.1:9776] received block 124, id 4ad5dddb65b2253e..., latency: -110ms, head 123
debug 2024-03-06T15:06:06.889 net-2     net_plugin.cpp:3761           handle_message       ] ["localhost:9776:blk - b741d54" - 1 127.0.0.1:9776] posting block 124 to dispatcher strand
info  2024-03-06T15:06:06.889 net-2     net_plugin.cpp:3788           operator()           ] bad block exception connection 1: #124 4ad5dddb65b2253e...: Block is not signed with expected key (3030008)
block signed by unexpected key