[txrequest] GetCandidatePeers

Needed for a later commit adding logic to ask the TxRequestTracker for a
list of announcers.  These announcers should know the parents of the
transaction they announced.

src/txrequest.h

@@ -195,6 +195,9 @@ public:
     /** Count how many announcements are being tracked in total across all peers and transaction hashes. */
     size_t Size() const;
 
+    /** For some tx return all peers with non-COMPLETED announcements for its txid or wtxid. The resulting vector may contain duplicate NodeIds. */
+    std::vector<NodeId> GetCandidatePeers(const CTransactionRef& tx) const;
+
     /** Access to the internal priority computation (testing only) */
     uint64_t ComputePriority(const uint256& txhash, NodeId peer, bool preferred) const;