Add a 'share' parameter to wait_for_message (default true, preserving
existing behavior). When share=false, each handler invocation requires
its own dedicated message to proceed — a message consumed by one
handler is not visible to others.
Use share=false for the pause_before_barrier_and_drain injection in
raft_topology_cmd_handler. The topology coordinator sends multiple
barrier_and_drain RPCs during a single topology transition (one per
state change). With share=true a single message_injection call
releases all handlers. With share=false the test can release them
one at a time, controlling exactly which topology state the write
handler's ERM captures.
324a08295d