[rpc] add unbroadcast info to mempool entries and getmempoolinfo

- expose info about number of txns in unbroadcast set and whether a mempool entry's tx has passed initial broadcast
- makes rpcs more informative and allows for more explicit testing, eg tracking if tx is in unbroadcast set
before and after originating node connects to peers (adds this in mempool_unbroadcast.py)
- adds mempool method IsUnbroadcastTx to query for tx inclusion in  mempool's unbroadcast set

src/rpc/blockchain.cpp

@@ -399,6 +399,7 @@ static std::vector<RPCResult> MempoolEntryDescription() { return {
     RPCResult{RPCResult::Type::ARR, "spentby", "unconfirmed transactions spending outputs from this transaction",
         {RPCResult{RPCResult::Type::STR_HEX, "transactionid", "child transaction id"}}},
     RPCResult{RPCResult::Type::BOOL, "bip125-replaceable", "Whether this transaction could be replaced due to BIP125 (replace-by-fee)"},
+    RPCResult{RPCResult::Type::BOOL, "unbroadcast", "Whether this transaction is currently unbroadcast (initial broadcast not yet confirmed)"},
 };}
 
 static void entryToJSON(const CTxMemPool& pool, UniValue& info, const CTxMemPoolEntry& e) EXCLUSIVE_LOCKS_REQUIRED(pool.cs)
@@ -460,6 +461,7 @@ static void entryToJSON(const CTxMemPool& pool, UniValue& info, const CTxMemPool
     }
 
     info.pushKV("bip125-replaceable", rbfStatus);
+    info.pushKV("unbroadcast", pool.IsUnbroadcastTx(tx.GetHash()));
 }
 
 UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose)
@@ -1389,7 +1391,7 @@ UniValue MempoolInfoToJSON(const CTxMemPool& pool)
     ret.pushKV("maxmempool", (int64_t) maxmempool);
     ret.pushKV("mempoolminfee", ValueFromAmount(std::max(pool.GetMinFee(maxmempool), ::minRelayTxFee).GetFeePerK()));
     ret.pushKV("minrelaytxfee", ValueFromAmount(::minRelayTxFee.GetFeePerK()));
-
+    ret.pushKV("unbroadcastcount", uint64_t{pool.GetUnbroadcastTxs().size()});
     return ret;
 }
 
@@ -1408,6 +1410,7 @@ static UniValue getmempoolinfo(const JSONRPCRequest& request)
                         {RPCResult::Type::NUM, "maxmempool", "Maximum memory usage for the mempool"},
                         {RPCResult::Type::STR_AMOUNT, "mempoolminfee", "Minimum fee rate in " + CURRENCY_UNIT + "/kB for tx to be accepted. Is the maximum of minrelaytxfee and minimum mempool fee"},
                         {RPCResult::Type::STR_AMOUNT, "minrelaytxfee", "Current minimum relay fee for transactions"},
+                        {RPCResult::Type::NUM, "unbroadcastcount", "Current number of transactions that haven't passed initial broadcast yet"}
                     }},
                 RPCExamples{
                     HelpExampleCli("getmempoolinfo", "")