Use boost::asio::deadline_timer for walletpassphrase timeout

New method in bitcoinrpc:  RunLater, that uses a map of deadline
timers to run a function later.

Behavior of walletpassphrase is changed; before, calling
walletpassphrase again before the lock timeout passed
would result in: Error: Wallet is already unlocked.

You would have to call lockwallet before walletpassphrase.

Now: the last walletpassphrase with correct password
wins, and overrides any previous timeout.

Fixes issue# 1961 which was caused by spawning too many threads.

Test plan:

Start with encrypted wallet, password 'foo'
NOTE:
 python -c 'import time; print("%d"%time.time())'
... will tell you current unix timestamp.

Try:

walletpassphrase foo 600
getinfo
EXPECT: unlocked_until is about 10 minutes in the future

walletpassphrase foo 1
sleep 2
sendtoaddress mun74Bvba3B1PF2YkrF4NsgcJwHXXh12LF 11
EXPECT: Error: Please enter the wallet passphrase with walletpassphrase first.

walletpassphrase foo 600
walletpassphrase foo 0
getinfo
EXPECT: wallet is locked (unlocked_until is 0)

walletpassphrase foo 10
walletpassphrase foo 600
getinfo
EXPECT: wallet is unlocked until 10 minutes in future

walletpassphrase foo 60
walletpassphrase bar 600
EXPECT: Error, incorrect passphrase
getinfo
EXPECT: wallet still scheduled to lock 60 seconds from first (successful) walletpassphrase

src/wallet.cpp

@@ -87,9 +87,6 @@ bool CWallet::AddCScript(const CScript& redeemScript)
 
 bool CWallet::Unlock(const SecureString& strWalletPassphrase)
 {
-    if (!IsLocked())
-        return false;
-
     CCrypter crypter;
     CKeyingMaterial vMasterKey;
 
@@ -100,7 +97,7 @@ bool CWallet::Unlock(const SecureString& strWalletPassphrase)
             if(!crypter.SetKeyFromPassphrase(strWalletPassphrase, pMasterKey.second.vchSalt, pMasterKey.second.nDeriveIterations, pMasterKey.second.nDerivationMethod))
                 return false;
             if (!crypter.Decrypt(pMasterKey.second.vchCryptedKey, vMasterKey))
-                return false;
+                continue; // try another master key
             if (CCryptoKeyStore::Unlock(vMasterKey))
                 return true;
         }