The relaxed read of the step-down timestamp is spelled out in full at roughly fourteen call sites, which is verbose and repetitive. Add two inline helpers and convert the call sites:

• __wt_disagg_step_down_timestamp(session) - relaxed read, returns the timestamp.
• __wt_disagg_stepping_down(session) - returns true when the timestamp is set.

Call sites: schema_worker.c, schema_drop.c, schema_create.c, conn_layered.c, cur_layered.c, evict_inline.h, txn.c, txn_inline.h, session_api.c, txn_timestamp.c.

Note that evict_inline.h is included before txn_inline.h in wt_internal.h, so the helpers' home header has to be chosen accordingly.