MOVEONLY: Expose BanMapToJson / BanMapFromJson

CSubNet serialization code that was removed in
fa4e6afdae was needed by multiprocess code
to share ban map between gui and node processes.

Rather than adding it back, use suggestion from MarcoFalke
<falke.marco@gmail.com>
https://github.com/bitcoin/bitcoin/pull/10102#discussion_r690922929 to
use JSON serialization. This requires making BanMapToJson /
BanMapFromJson functions public.

src/Makefile.am

@@ -548,6 +548,7 @@ libbitcoin_common_a_SOURCES = \
   key.cpp \
   key_io.cpp \
   merkleblock.cpp \
+  net_types.cpp \
   netaddress.cpp \
   netbase.cpp \
   net_permissions.cpp \