httpserver: explicitly detach worker threads

When using std::thread in place of boost::thread, letting the threads destruct
results in a std::terminate. According to the docs, the same thing should be
be happening in later boost versions:
http://www.boost.org/doc/libs/1_55_0/doc/html/thread/thread_management.html#thread.thread_management.thread.destructor

I'm unsure why this hasn't blown up already, but explicitly detaching can't
hurt.

Zcash: cherry-picked from commit d3773ca9aeb0d2f12dc0c5a0726778050c8cb455
This fixes #2554 (zcash-cli stop during getblocktemplate long poll
causes 'Assertion `!pthread_mutex_unlock(&m)' failed.')

src/httpserver.cpp

@@ -447,8 +447,10 @@ bool StartHTTPServer()
     LogPrintf("HTTP: starting %d worker threads\n", rpcThreads);
     threadHTTP = boost::thread(boost::bind(&ThreadHTTP, eventBase, eventHTTP));
 
-    for (int i = 0; i < rpcThreads; i++)
-        boost::thread(boost::bind(&HTTPWorkQueueRun, workQueue));
+    for (int i = 0; i < rpcThreads; i++) {
+        boost::thread rpc_worker(HTTPWorkQueueRun, workQueue);
+        rpc_worker.detach();
+    }
     return true;
 }