JetStream first pass basics.

This is the first checkin for JetStream. Has some rudimentary basics working.

TODO
1. Push vs pull mode for observables. (work queues)
2. Disk/File store, memory only for now.
3. clustering code - design shaping up well.
4. Finalize account import semantics.
5. Lots of other little things.

Signed-off-by: Derek Collison <derek@nats.io>

server/sysmem/mem_windows.go

@@ -0,0 +1,44 @@
+// Copyright 2019 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build windows
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/ns-sysinfoapi-memorystatusex
+type _memoryStatusEx struct {
+	dwLength     uint32
+	dwMemoryLoad uint32
+	ullTotalPhys uint64
+	unused       [6]uint64 // ignore rest of struct
+}
+
+func Memory() int64 {
+	k32, err := syscall.LoadDLL("kernel32.dll")
+	if err != nil {
+		return 0
+	}
+	globalMemoryStatusEx, err := kernel32.FindProc("GlobalMemoryStatusEx")
+	if err != nil {
+		return 0
+	}
+	msx := &_memoryStatusEx{dwLength: 64}
+	res, _, _ := globalMemoryStatusEx.Call(uintptr(unsafe.Pointer(msx)))
+	if res == 0 {
+		return 0
+	}
+	return msx.ullTotalPhys
+}