Split ::HEADERS processing into two separate cs_main locks

This will allow NotifyHeaderTip to be called from an
AcceptBlockHeader wrapper function without holding cs_main.

src/main.cpp

@@ -5982,14 +5982,14 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             ReadCompactSize(vRecv); // ignore tx count; assume it is 0.
         }
 
-        {
-        LOCK(cs_main);
-
         if (nCount == 0) {
             // Nothing interesting. Stop asking this peers for more headers.
             return true;
         }
 
+        CBlockIndex *pindexLast = NULL;
+        {
+        LOCK(cs_main);
         CNodeState *nodestate = State(pfrom->GetId());
 
         // If this looks like it could be a block announcement (nCount <
@@ -6019,7 +6019,6 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             return true;
         }
 
-        CBlockIndex *pindexLast = NULL;
         BOOST_FOREACH(const CBlockHeader& header, headers) {
             CValidationState state;
             if (pindexLast != NULL && header.hashPrevBlock != pindexLast->GetBlockHash()) {
@@ -6035,7 +6034,11 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                 }
             }
         }
+        }
 
+        {
+        LOCK(cs_main);
+        CNodeState *nodestate = State(pfrom->GetId());
         if (nodestate->nUnconnectingHeaders > 0) {
             LogPrint("net", "peer=%d: resetting nUnconnectingHeaders (%d -> 0)\n", pfrom->id, nodestate->nUnconnectingHeaders);
         }