[wallet] Use global g_wallet_init_interface to init/destroy the wallet.

This commit creates a global g_wallet_init_interface, which is created
in bitcoind and bitcoin-qt. g_wallet_init_interface is used to init
and destroy the wallet.

This removes the dependency from init.cpp on the wallet library.

src/bitcoind.cpp

@@ -18,6 +18,10 @@
 #include <httpserver.h>
 #include <httprpc.h>
 #include <utilstrencodings.h>
+#if ENABLE_WALLET
+#include <wallet/init.h>
+#include <walletinitinterface.h>
+#endif
 
 #include <boost/thread.hpp>
 
@@ -59,6 +63,10 @@ bool AppInit(int argc, char* argv[])
 {
     bool fRet = false;
 
+#if ENABLE_WALLET
+    g_wallet_init_interface.reset(new WalletInit);
+#endif
+
     //
     // Parameters
     //