Set default RPC port to 60557

[albrow]

There is currently a discrepancy in the documentation regarding the default RPC port. See https://0x-org.gitbook.io/mesh/v/v6.0.1-beta/getting-started/deployment.

In part of the documentation we claim:

Ports 60557, 60558, and 60559 are the default ports used for the JSON RPC endpoint, communicating with peers over TCP, and communicating with peers over WebSockets, respectively.

However, later the documentation conflicts with this when the RPC_ADDR environment variable itself is documented:

// RPCAddr is the interface and port to use for the JSON-RPC API over
// WebSockets. By default, 0x Mesh will listen on localhost and will let the
// OS select a randomly available port.
RPCAddr string envvar:"RPC_ADDR" default:"localhost:0"

In version 6.0.1-beta, Mesh randomly selected an available port by default. So the second part of the docs was correct.

After thinking about this some more, I actually think it makes more sense to use 60557 as the default port, as we already suggest in the docker-compose.yml file for enabling telemetry. This PR updates the default RPC_ADDR to use port 60557 and also updates the relevant portions of our documentation so that everything is consistent.

[fabioberger]

Please add a CHANGELOG entry for this.