kernel: Move MessageStartChars to its own file

The protocol.h file contains many non-consensus related definitions and
should thus not be part of the libbitcoinkernel. This commit makes
protocol.h no longer a required include for users of the
libbitcoinkernel.

This commit is part of the libbitcoinkernel project, namely its stage 1
step 3: Decouple most non-consensus headers from libbitcoinkernel.

Co-Authored-By: Cory Fields <cory-nospam-@coryfields.com>

src/validation.cpp

@@ -23,6 +23,7 @@
 #include <hash.h>
 #include <kernel/chainparams.h>
 #include <kernel/mempool_entry.h>
+#include <kernel/messagestartchars.h>
 #include <kernel/notifications_interface.h>
 #include <logging.h>
 #include <logging/timer.h>
@@ -4533,7 +4534,7 @@ void ChainstateManager::LoadExternalBlockFile(
             unsigned int nSize = 0;
             try {
                 // locate a header
-                CMessageHeader::MessageStartChars buf;
+                MessageStartChars buf;
                 blkdat.FindByte(std::byte(params.MessageStart()[0]));
                 nRewind = blkdat.GetPos() + 1;
                 blkdat >> buf;