assertion on iproto drop connections/shutdown race with new connection

[nshy]

Found in 3.1.0-entrypoint-115-g1fb98714e (3.0 is also affected)

The recently introduced test is flaky under load:

./test-run.py --builddir ../../build-dev/tarantool `yes enterprise-luatest/security_test | head -100` -p test_iproto_drop_connections_basic

[016] Tarantool version is 3.1.0-entrypoint-48-g87952e9
[016] TAP version 13
[016] 1..1
[016] # Started on Tue Feb 20 14:51:10 2024
[016] # Starting group: iproto
[016] server | tarantool: ./tarantool/src/box/iproto.cc:3354: void net_send_greeting(cmsg*): Assertion `con->state == IPROTO_CONNECTION_ALIVE'
[016] server | failed.
[016] not ok 1	iproto.test_iproto_drop_connections_basic
[016] #   ...ool-ee/tarantool/test-run/lib/luatest/luatest/server.lua:409: Process is terminated when waiting for "server is ready" condition for server (alias: server, workdir: server-stz9cqzezpOP, pid: 1475456)
[016] #   stack traceback:
[016] #   	...l-ee/tarantool/test/enterprise-luatest/security_test.lua:760: in function 'iproto.test_iproto_drop_connections_basic'
[016] #   	...
[016] #   	[C]: in function 'xpcall'
[016] #   artifacts:
[016] #   	server -> /tmp/t/016_enterprise-luatest/artifacts/server-stz9cqzezpOP
[016] # Ran 1 tests in 1.463 seconds, 0 succeeded, 1 errored, 14 not selected

Looks like this is a race on new connection with iproto_drop_connections() which is introduced in 3.0.