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/bitcoinrpc.h

@@ -88,6 +88,12 @@ void RPCTypeCheck(const json_spirit::Array& params,
 void RPCTypeCheck(const json_spirit::Object& o,
                   const std::map<std::string, json_spirit::Value_type>& typesExpected, bool fAllowNull=false);
 
+/*
+  Run func nSeconds from now. Uses boost deadline timers.
+  Overrides previous timer <name> (if any).
+ */
+void RPCRunLater(const std::string& name, boost::function<void(void)> func, int64 nSeconds);
+
 typedef json_spirit::Value(*rpcfn_type)(const json_spirit::Array& params, bool fHelp);
 
 class CRPCCommand