Restore RPC HTTP keepalives to default.

This avoids a regression for issues like #334 where high speed
 repeated connections eventually run the HTTP client out of
 sockets because all of theirs end up in time_wait.

Maybe the trade-off here is suboptimal, but if both choices will
 fail then we prefer fewer changes until the root cause is solved.

src/init.cpp

@@ -380,7 +380,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += "  -rpcport=<port>        " + strprintf(_("Listen for JSON-RPC connections on <port> (default: %u or testnet: %u)"), 8332, 18332) + "\n";
     strUsage += "  -rpcallowip=<ip>       " + _("Allow JSON-RPC connections from specified source. Valid for <ip> are a single IP (e.g. 1.2.3.4), a network/netmask (e.g. 1.2.3.4/255.255.255.0) or a network/CIDR (e.g. 1.2.3.4/24). This option can be specified multiple times") + "\n";
     strUsage += "  -rpcthreads=<n>        " + strprintf(_("Set the number of threads to service RPC calls (default: %d)"), 4) + "\n";
-    strUsage += "  -rpckeepalive          " + strprintf(_("RPC support for HTTP persistent connections (default: %d)"), 0) + "\n";
+    strUsage += "  -rpckeepalive          " + strprintf(_("RPC support for HTTP persistent connections (default: %d)"), 1) + "\n";
 
     strUsage += "\n" + _("RPC SSL options: (see the Bitcoin Wiki for SSL setup instructions)") + "\n";
     strUsage += "  -rpcssl                                  " + _("Use OpenSSL (https) for JSON-RPC connections") + "\n";

src/rpcserver.cpp

@@ -953,7 +953,7 @@ void ServiceConnection(AcceptedConnection *conn)
         ReadHTTPMessage(conn->stream(), mapHeaders, strRequest, nProto, MAX_SIZE);
 
         // HTTP Keep-Alive is false; close connection immediately
-        if ((mapHeaders["connection"] == "close") || (!GetBoolArg("-rpckeepalive", false)))
+        if ((mapHeaders["connection"] == "close") || (!GetBoolArg("-rpckeepalive", true)))
             fRun = false;
 
         // Process via JSON-RPC API