refactor: drop protocol.h include header in rpc/util.h

as it was only needed for GetServicesNames(). This potentially avoids needlessly
compiling the 500 lines of protocol.h in the 35 files other than rpc/net.cpp
that include rpc/util.h.

Drop an unneeded CPubKey forward declaration. The other IWYU suggestions would
require more extensive changes in other files.

Add 3 already-missing include headers in other translation units that are needed
to compile without protocol.h in rpc/util.h, as it includes netaddress.h, which
in turn includes util/strencodings.h.

src/rpc/mempool.cpp

@@ -21,6 +21,7 @@
 #include <univalue.h>
 #include <util/fs.h>
 #include <util/moneystr.h>
+#include <util/strencodings.h>
 #include <util/time.h>
 
 #include <utility>