Simplify declaration and use of vars

This issue is a proposal to rationalize our use of NetSender vars.

Revid's are defined in:
`av/revid/config/variables.go`

Speaker's are defined in:
`av/cmd/speaker/main.go`

They need to be used by clients and cloud alike.

1. I don't think they belong in av, as they're client/device properties.
2. We have a lot of cookie-cutter code for parsing these vars, such as.

```
{
        Name:     KeyPSITime,
        Type:     typeUint,
        Update:   func(c *Config, v string) { c.PSITime = parseUint(KeyPSITime, v, c) },
        Validate: func(c *Config) { c.PSITime = lessThanOrEqual(KeyPSITime, c.PSITime, 0, c, defaultPSITime) },
},
{
        Name:   KeyQuantization,
        Type:   typeUint,
        Update: func(c *Config, v string) { c.Quantization = parseUint(KeyQuantization, v, c) },
},
}
```

I propose moving to a data-driven approach where vars and their types are represented in JSON and parsed by a general-purpose parser. Using reflection, we could map JSON keys onto config struct fields.