[wallet] Remove locked_chain from CWallet, its RPCs and tests

This change is intended to make the bitcoin node and its rpc, network
and gui interfaces more responsive while the wallet is in use. Currently
because the node's cs_main mutex is always locked before the wallet's
cs_wallet mutex (to prevent deadlocks), cs_main currently stays locked
while the wallet does relatively slow things like creating and listing
transactions.

This commit only remmove chain lock tacking in wallet code, and invert
lock order from cs_main, cs_wallet to cs_wallet, cs_main.
must happen at once to avoid any deadlock. Previous commit were only
removing Chain::Lock methods to Chain interface and enforcing they
take cs_main.

Remove LockChain method from CWallet and Chain::Lock interface.

src/interfaces/wallet.h

@@ -300,6 +300,9 @@ public:
     //! Register handler for keypool changed messages.
     using CanGetAddressesChangedFn = std::function<void()>;
     virtual std::unique_ptr<Handler> handleCanGetAddressesChanged(CanGetAddressesChangedFn fn) = 0;
+
+    //! Return pointer to internal wallet class, useful for testing.
+    virtual CWallet* wallet() { return nullptr; }
 };
 
 //! Information about one wallet address.