vcsim: add support for PropertyCollector incremental updates

[dougm]

Prior to this change, clients could only call WaitForUpdatesEx once,
which would return the current state. The next invocation with Version
given would fail as unsupported. Clients can now use WaitForUpdatesEx
to wait for multiple updates.

The basic idea is that methods should now update object fields using
the Registry.Update method instead of setting fields directly.
Registry.Update uses the same PropertyChange structure that is sent as
part of the WaitForUpdatesEx response. Registry.Update will also apply
the PropertyChange(s) to the local object, removing the need to set
fields directly.

• govc commands now use Client.Flag.WithCancel to catch SIGINT,
  allowing proper cleanup of collectors

• Add vcsim ListView support to ViewManager

Fixes #922

[dougm]

Still some tests and doc to add, but the basics are functioning now. There are still methods that are setting object fields directly, we can convert more to using UpdateObject in separate PRs.
The property path filters also still need to be applied, currently all updates are included in the response, though the objects themselves are properly filtered.

A few examples that now work against vcsim:

% govc tasks -f

% govc events -f

% govc object.collect -wait 1h -type m / runtime.powerState

[dougm]

cc @ausevj @hickeng @kars7e @embano1

[dougm]

Added more tests and property path filters since opening the PR, ready for review now.

[jiatongw]

sig := make(chan os.Signal, 1)?
https://gobyexample.com/signals as a reference

[dougm]

That reference doesn't say why though. Go's docs are a bit better: https://golang.org/pkg/os/signal/#Notify
Your comment here also made me realize the sig channel needs to be created earlier, before the go routine calling f is started. Changed so that happens first.

[jiatongw]

Here is the tricky part, about t.Fatal() in goroutine. I have read similar code before when I was learning Golang and I searched online. There is a group discussion: Whenever t.Fatal is called in a goroutine, it is swallowed and lost. (reference https://groups.google.com/forum/#!topic/vault-tool/zNlHJLxoo3w, also here ipfs/kubo#2043). One of their guys also used staticcheck to check their code. So, for double check, I also ran it against this file. Result is the same as theirs. So, we can discuss about that.
Line 119, 126, 131, 152

[dougm]

Good catch, I've changed those to use t.Error instead.

[jiatongw]

Generally, I pulled your PR and ran some commands, it works well.

[jiatongw]

@dougm Travis CI build failed here. simulator/property_collector_test.go line 404, 442 also need to change to t.Error

[dougm]

Yeah I've been re-running the Travis build manually as the race detector is finding some issues that I don't see locally. I'll fix the Fatal -> Error logs.

[jiatongw]

Looks good to me 👍