[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/init.h

@@ -6,11 +6,15 @@
 #ifndef BITCOIN_INIT_H
 #define BITCOIN_INIT_H
 
+#include <memory>
 #include <string>
 
 class CScheduler;
 class CWallet;
 
+class WalletInitInterface;
+extern std::unique_ptr<WalletInitInterface> g_wallet_init_interface;
+
 namespace boost
 {
 class thread_group;