Implement client-server interaction via @server and @browser-session

[ibdknox]

This adds a new kind of database that attempts to send its adds over the socket to either the client or the server depending on where it's running. It also adds the --clientAndServer flag, which allows you to run your program both on the client and server.

[ibdknox]

Example program I used to test:

## Main screen
```
search
  [#active-user name]

bind @browser
  [#div text: "Hello, {{name}}"]
```

## Login screen
Login screen is displayed when there is no active user:
```eve
search
  not([#active-user])

bind @browser
  [#div children:
    [#div children:
      [#label text: "Username:"]
      [#input #user-name type: "text"]]
    [#div children:
      [#label text: "Password:"]
      [#input #user-password type: "password"]]
    [#button #user-submit text: "Login"]]
```

Currently, submitting the form automatically succeeds:
```
search @browser @event
  [#click element: [#button #user-submit]]
  [#user-name value]

commit
  [#active-user name: value]
commit @server
  [#foo name: value]
```

```
search @server
  [#foo name]
commit @browser-session
  [#bar name]
```

```
search @browser-session
  [#bar name]
bind @browser
  [#div text: "hey {{name}} from the server"]
```

[joshuafcole]

Everything looks good. It raises a few questions about how this should interact with other aspects of the language, but so long as we mark this guy as being prototype only I don't see that as being blocking.

[joshuafcole]

are there any other options here? If not both is probably fine, but if so we might want to be more explicit.

[joshuafcole]

Yeah, we need to figure out how the extensibility story and policy story interact to give us this if we think it's important. This isn't blocking but it merits a research task.

[joshuafcole]

We could potentially make client-server equipped code run on either/or as well if we always add this and just let it play telephone with itself. I don't know that doing so is particularly useful, but it may be good for testing at least.

[ibdknox]

I don't think this is the "right" solution moving forward, so closing this out for now.