assumeUTXO: fix peers disconnection during sync

Because AssumeUTXO nodes prioritize tip synchronization, they relay their local
address through the network before completing the background chain sync.
This, combined with the advertising of full-node service (NODE_NETWORK), can
result in an honest peer in IBD connecting to the AssumeUTXO node (while syncing)
and requesting an historical block the node does not have. This behavior leads to
an abrupt disconnection due to perceived unresponsiveness (lack of response)
from the AssumeUTXO node.

This lack of response occurs because nodes ignore getdata requests when they do
not have the block data available (further discussion can be found in PR 30385).

Fix this by refraining from signaling full-node service support while the
background chain is being synced. During this period, the node will only
signal 'NODE_NETWORK_LIMITED' support. Then, full-node ('NODE_NETWORK')
support will be re-enabled once the background chain sync is completed.

src/validation.cpp

@@ -3575,8 +3575,8 @@ bool Chainstate::ActivateBestChain(BlockValidationState& state, std::shared_ptr<
             //
             // This cannot be done while holding cs_main (within
             // MaybeCompleteSnapshotValidation) or a cs_main deadlock will occur.
-            if (m_chainman.restart_indexes) {
-                m_chainman.restart_indexes();
+            if (m_chainman.snapshot_download_completed) {
+                m_chainman.snapshot_download_completed();
             }
             break;
         }