Remove uses of cs_main in wallet code

This commit does not change behavior.

It is easiest to review this change with:

    git log -p -n1 -U0

src/interfaces/chain.cpp

@@ -4,13 +4,37 @@
 
 #include <interfaces/chain.h>
 
+#include <sync.h>
 #include <util/system.h>
+#include <validation.h>
+
+#include <memory>
+#include <utility>
 
 namespace interfaces {
 namespace {
 
+class LockImpl : public Chain::Lock
+{
+};
+
+class LockingStateImpl : public LockImpl, public UniqueLock<CCriticalSection>
+{
+    using UniqueLock::UniqueLock;
+};
+
 class ChainImpl : public Chain
 {
+public:
+    std::unique_ptr<Chain::Lock> lock(bool try_lock) override
+    {
+        auto result = MakeUnique<LockingStateImpl>(::cs_main, "cs_main", __FILE__, __LINE__, try_lock);
+        if (try_lock && result && !*result) return {};
+        // std::move necessary on some compilers due to conversion from
+        // LockingStateImpl to Lock pointer
+        return std::move(result);
+    }
+    std::unique_ptr<Chain::Lock> assumeLocked() override { return MakeUnique<LockImpl>(); }
 };
 
 } // namespace