Separate Bot and CAS service access points for bots.

Context

A bots accesses two different services: the Bots and ContentAddressableStorage (and/or ByteStream) services. At the moment, only the buildbox bot support using two different end-points for these two different services. Every bot implementation should.

Task Description

• Add new, CAS specific, CLI options for CAS remote and TLS keys.
• Factorise the channel/stub creation code.
• Remove the buildbox specific bits.