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, 2 insertions, 2 deletions

diff --git a/src/validationinterface.h b/src/validationinterface.h
index b21b6e5782..cb261f6aa3 100644
--- a/src/validationinterface.h
+++ b/src/validationinterface.h
@@ -33,7 +33,7 @@ protected:
     virtual void SetBestChain(const CBlockLocator &locator) {};
     virtual void UpdatedTransaction(const uint256 &hash) {};
     virtual void Inventory(const uint256 &hash) {};
-    virtual void ResendWalletTransactions() {};
+    virtual void ResendWalletTransactions(int64_t nBestBlockTime) {};
     virtual void BlockChecked(const CBlock&, const CValidationState&) {};
     friend void ::RegisterValidationInterface(CValidationInterface*);
     friend void ::UnregisterValidationInterface(CValidationInterface*);
@@ -52,7 +52,7 @@ struct CMainSignals {
     /** Notifies listeners about an inventory item being seen on the network. */
     boost::signals2::signal<void (const uint256 &)> Inventory;
     /** Tells listeners to broadcast their data. */
-    boost::signals2::signal<void ()> Broadcast;
+    boost::signals2::signal<void (int64_t nBestBlockTime)> Broadcast;
     /** Notifies listeners of a block validation result */
     boost::signals2::signal<void (const CBlock&, const CValidationState&)> BlockChecked;
 };