wallet: cancellable refresh

^C while in manual refresh will cancel the refresh, since that's
often an annoying thing to have to wait for. Also, a manual refresh
command will interrupt any running background refresh and take
over, rather than wait for the background refresh to be done, and
look to be hanging.

src/wallet/wallet2.cpp

@@ -680,6 +680,7 @@ void wallet2::refresh(uint64_t start_height, uint64_t & blocks_fetched, bool& re
   get_short_chain_history(short_chain_history);
   pull_blocks(start_height, blocks_start_height, short_chain_history, blocks);
 
+  m_run.store(true, std::memory_order_relaxed);
   while(m_run.load(std::memory_order_relaxed))
   {
     try