This is an advanced feature which will disable any kind of automatic
transaction broadcasting in the wallet. This gives the user full control
of how the transaction is sent.
For example they can broadcast new transactions through some other
mechanism themselves, after getting the transaction hex through `gettransaction`.
This just adds the option `-walletbroadcast=<0,1>`. Right now these
transactions will get the status
Status: conflicted, has not been successfully broadcast yet
They shouldn't be shown as conflicted at all (`walletconflicts` is empty). This status
will go away when the transaction is received through the network.
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.
