Regression test for ResendWalletTransactions

Adds a regression test for the wallet's ResendWalletTransactions function, which uses a new, hidden RPC command "resendwallettransactions."

I refactored main's Broadcast signal so it is passed the best-block time, which let me remove a global variable shared between main.cpp and the wallet (nTimeBestReceived).

I also manually tested the "rebroadcast unconfirmed every half hour or so" functionality by:

1. Running bitcoind -connect=0.0.0.0:8333
2. Creating a couple of send-to-self transactions
3. Connect to a peer using -addnode
4. Waited a while, monitoring debug.log, until I see:
```2015-03-23 18:48:10 ResendWalletTransactions: rebroadcast 2 unconfirmed transactions```

One last change: don't bother putting ResendWalletTransactions messages in debug.log unless unconfirmed transactions were actually rebroadcast.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 4a13f02195..6ae1c87b1d 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -381,7 +381,7 @@ public:
     int64_t GetTxTime() const;
     int GetRequestCount() const;
 
-    void RelayWalletTransaction();
+    bool RelayWalletTransaction();
 
     std::set<uint256> GetConflicts() const;
 };
@@ -614,7 +614,8 @@ public:
     void EraseFromWallet(const uint256 &hash);
     int ScanForWalletTransactions(CBlockIndex* pindexStart, bool fUpdate = false);
     void ReacceptWalletTransactions();
-    void ResendWalletTransactions();
+    void ResendWalletTransactions(int64_t nBestBlockTime);
+    std::vector<uint256> ResendWalletTransactionsBefore(int64_t nTime);
     CAmount GetBalance() const;
     CAmount GetUnconfirmedBalance() const;
     CAmount GetImmatureBalance() const;