wallet2_api: add an address book payment id lookup API

src/wallet/api/address_book.cpp

@@ -103,6 +103,28 @@ bool AddressBookImpl::deleteRow(std::size_t rowId)
   return r;
 } 
 
+int AddressBookImpl::lookupPaymentID(const std::string &payment_id) const
+{
+    // turn short ones into long ones for comparison
+    const std::string long_payment_id = payment_id + std::string(64 - payment_id.size(), '0');
+
+    int idx = -1;
+    for (const auto &row: m_rows) {
+        ++idx;
+        // this does short/short and long/long
+        if (payment_id == row->getPaymentId())
+            return idx;
+        // short/long
+        if (long_payment_id == row->getPaymentId())
+            return idx;
+        // one case left: payment_id was long, row's is short
+        const std::string long_row_payment_id = row->getPaymentId() + std::string(64 - row->getPaymentId().size(), '0');
+        if (payment_id == long_row_payment_id)
+            return idx;
+    }
+    return -1;
+}
+
 void AddressBookImpl::clearRows() {
    for (auto r : m_rows) {
      delete r;

src/wallet/api/address_book.h

@@ -51,6 +51,8 @@ public:
     // Error codes. See AddressBook:ErrorCode enum in wallet2_api.h
     std::string errorString() const {return m_errorString;}
     int errorCode() const {return m_errorCode;}
+
+    int lookupPaymentID(const std::string &payment_id) const;
     
 private:
     void clearRows();

src/wallet/wallet2_api.h

@@ -175,6 +175,7 @@ struct AddressBook
     virtual void refresh() = 0;  
     virtual std::string errorString() const = 0;
     virtual int errorCode() const = 0;
+    virtual int lookupPaymentID(const std::string &payment_id) const = 0;
 };
 
 struct WalletListener