net: move cs_main up in FetchBlock to fix rpc assert crash

1. FetchBlock runs in a http worker thread. It acquires a PeerRef,
locks cs_main, then may later call BlockRequested which asserts
that CNodeState exists for the peer.

2. FinalizeNode may run in either the bitcoind or b-net threads. It
locks cs_main, fetches a PeerRef from RemovePeer, fetches a CNodeState,
and later removes it from m_node_states.

Because of the lock placement in FetchBlock, the http worker thread in 1)
can acquire a valid PeerRef and block while the b-net thread in 2) is
cleaning up the peer in FinalizeNode. When the worker thread later acquires
cs_main, it may crash in BlockRequested since no CNodeState exists. Fix
this by acquiring the lock earlier in FetchBlock.

The lock can be replaced with a net-specific lock when the remaining
CNodeState fields are moved to Peer.

src/net_processing.cpp

@@ -1977,6 +1977,12 @@ util::Expected<void, std::string> PeerManagerImpl::FetchBlock(NodeId peer_id, co
 {
     if (m_chainman.m_blockman.LoadingBlocks()) return util::Unexpected{"Loading blocks ..."};
 
+    // The lock must be taken here before fetching Peer so another thread does
+    // not delete the CNodeState from under the current thread, causing an
+    // assertion failure in BlockRequested. This lock can be replaced with a
+    // net-specific lock when more of CNodeState is moved into Peer.
+    LOCK(cs_main);
+
     // Ensure this peer exists and hasn't been disconnected
     PeerRef peer = GetPeerRef(peer_id);
     if (peer == nullptr) return util::Unexpected{"Peer does not exist"};
@@ -1984,8 +1990,6 @@ util::Expected<void, std::string> PeerManagerImpl::FetchBlock(NodeId peer_id, co
     // Ignore pre-segwit peers
     if (!CanServeWitnesses(*peer)) return util::Unexpected{"Pre-SegWit peer"};
 
-    LOCK(cs_main);
-
     // Forget about all prior requests
     RemoveBlockRequest(block_index.GetBlockHash(), std::nullopt);