Docs: connector cleanups

[slfritchie]

Fix up various docs related to connector sources & sinks.

• connector-protocol-v3.md: fix misc errors and typos, clarify how a connector sink must respond to 2PC operations
• effectively-once/README.md: fix bitrot in use examples, add major section on master-crasher.sh script use,
• master-crasher.sh: small changes to adapt to new docs
• aloc_sink/README.md: new file
• aloc_sink scripts: minor changes