WT-11126 uses IDs for every key.  That means a lookup like

WT_ERR(__wt_conf_gets_def(session, conf, isolation, 0, &cval)) 

translates (via a macro) into a function call to look up integer id WT_CONF_ID_isolation.

y.ershov@mongodb.com made a great suggestion during the review that instead of declaring

  WT_CONF *conf

we could declare:

  WT_CONF_session_begin_transaction *conf

That structure could be something like:

  typedef struct {
       WT_CONFIG_ITEM isolation;
       WT_CONFIG_ITEM sync;
       struct {
          WT_CONFIG_ITEM prepared;
          WT_CONFIG_ITEM read;
      } roundup_timestamps;
  } WT_CONF_session_begin_transaction;

Then getting value amounts to:

   cval = conf->isolation;

This is all hidden by a macro, so the WT code remains as:

WT_ERR(__wt_conf_gets_def(session, conf, isolation, 0, &cval)) 

It's not entirely that simple, we do need to do something a bit different with bound arguments.

The only drawback I see is how we deal with sets of keys that are shared among multiple APIs. If there is a common function that parses the configs for say, open_session and session->reconfigure, how do we share that? I think there are solutions to this.