Draft: Re-define workflows when using replicas

What does this MR do?

This replaces other load balancing code with a set of clearly defined workflows:

• isolated_session: opens a new session with a new context
• read_only: we intent to only read data, disallowing writes
• isolated_write: we intent to perform isolated write

This removes all prior methods:

• without_sticky_writes
• use_replicas_for_read_queries
• fallback_to_replicas_for_ambiguous_queries

This ensures that operations are executed in a transactional way. We continue to be able to use the .use_primary! to work as it was.

Related:

• #330530

Conformity

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