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/sysctl.go

@@ -0,0 +1,31 @@
+// 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 darwin freebsd openbsd dragonfly netbsd
+
+package sysmem
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func sysctlInt64(name string) int64 {
+	s, err := syscall.Sysctl(name)
+	if err != nil {
+		return 0
+	}
+	// hack because the string conversion above drops a \0
+	b := []byte(s)
+	return *(*int64)(unsafe.Pointer(&b[0]))
+}