store secret keys encrypted where possible

The secret spend key is kept encrypted in memory, and
decrypted on the fly when needed.

Both spend and view secret keys are kept encrypted in a JSON
field in the keys file. This avoids leaving the keys in
memory due to being manipulated by the JSON I/O API.

src/wallet/api/wallet.cpp

@@ -2032,7 +2032,8 @@ bool WalletImpl::isNewWallet() const
 
 bool WalletImpl::doInit(const string &daemon_address, uint64_t upper_transaction_size_limit, bool ssl)
 {
-    if (!m_wallet->init(daemon_address, m_daemon_login, upper_transaction_size_limit, ssl))
+    // claim RPC so there's no in-memory encryption for now
+    if (!m_wallet->init(true, daemon_address, m_daemon_login, upper_transaction_size_limit, ssl))
        return false;
 
     // in case new wallet, this will force fast-refresh (pulling hashes instead of blocks)