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/node/blockstorage.cpp

@@ -11,6 +11,7 @@
 #include <flatfile.h>
 #include <hash.h>
 #include <kernel/chainparams.h>
+#include <kernel/messagestartchars.h>
 #include <logging.h>
 #include <pow.h>
 #include <reverse_iterator.h>
@@ -927,7 +928,7 @@ bool BlockManager::ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatF
     }
 
     try {
-        CMessageHeader::MessageStartChars blk_start;
+        MessageStartChars blk_start;
         unsigned int blk_size;
 
         filein >> blk_start >> blk_size;