index: decrease ThreadSync cs_main contention

Only NextSyncBlock requires cs_main lock. The
other function calls like Commit or Rewind will
lock or not cs_main internally when they need it.

Avoiding keeping cs_main locked when Commit() or
Rewind() write data to disk.

src/index/base.cpp

@@ -159,23 +159,20 @@ void BaseIndex::Sync()
                 return;
             }
 
-            {
-                LOCK(cs_main);
-                const CBlockIndex* pindex_next = NextSyncBlock(pindex, m_chainstate->m_chain);
-                if (!pindex_next) {
-                    SetBestBlockIndex(pindex);
-                    m_synced = true;
-                    // No need to handle errors in Commit. See rationale above.
-                    Commit();
-                    break;
-                }
-                if (pindex_next->pprev != pindex && !Rewind(pindex, pindex_next->pprev)) {
-                    FatalErrorf("%s: Failed to rewind index %s to a previous chain tip",
-                               __func__, GetName());
-                    return;
-                }
-                pindex = pindex_next;
+            const CBlockIndex* pindex_next = WITH_LOCK(cs_main, return NextSyncBlock(pindex, m_chainstate->m_chain));
+            if (!pindex_next) {
+                SetBestBlockIndex(pindex);
+                m_synced = true;
+                // No need to handle errors in Commit. See rationale above.
+                Commit();
+                break;
             }
+            if (pindex_next->pprev != pindex && !Rewind(pindex, pindex_next->pprev)) {
+                FatalErrorf("%s: Failed to rewind index %s to a previous chain tip", __func__, GetName());
+                return;
+            }
+            pindex = pindex_next;
+
 
             auto current_time{std::chrono::steady_clock::now()};
             if (last_log_time + SYNC_LOG_INTERVAL < current_time) {