Support multiple collections

[juntao]

Some applications have both short form QA and long form article contents in the knowledge base. To answer questions correctly, we should ideally have context from both sets. To support this, we should support multiple comma-delimited values for the following parameters.

• --qdrant-collection-name
• --qdrant-limit
• --qdrant-score-threshold

Ideally, the RAG server should issue multiple queries in parallel and adds results into the system prompt as soon as they all return.

This should be backward compatible -- that is the current single value arguments should still work.