Introduce new V4 format addrman

92617b7a75 effectively changed the
on-disk format in an incompatible way: old deserializers cannot
deal with multiple entries for the same IP.

Introduce a V4_MULTIPORT format, and increment the compatibility base,
so that old versions correctly recognize it as an incompatible future
version.

test/functional/feature_addrman.py

@@ -18,7 +18,7 @@ from test_framework.util import assert_equal
 def serialize_addrman(
     *,
     format=1,
-    lowest_compatible=3,
+    lowest_compatible=4,
     net_magic="regtest",
     bucket_key=1,
     len_new=None,
@@ -75,7 +75,7 @@ class AddrmanTest(BitcoinTestFramework):
             expected_msg=init_error(
                 "Unsupported format of addrman database: 1. It is compatible with "
                 "formats >=111, but the maximum supported by this version of "
-                f"{self.config['environment']['PACKAGE_NAME']} is 3.: (.+)"
+                f"{self.config['environment']['PACKAGE_NAME']} is 4.: (.+)"
             ),
             match=ErrorMatch.FULL_REGEX,
         )