Reconfigure cleanup

@agorrod, @sue -- this is the last piece of #1172.

Now we're merging the configuration strings, I changed the underlying reconfigure functions to error in the case of not-found being returned for a configuration string, because we're supposed to have all of them, all the time, now.

Sue, you agreed to do the review.  (I tried to keep the commits separate so it's not a monolithic chunk.)

Alex, I'm copying you on this one because there were a couple of functions I wasn't sure how to fix. Specifically, this chunk in `__wt_cache_config`:
```
/*
 * If not using a shared cache configure the cache size, otherwise
 * check for a reserved size.
 */
if (!F_ISSET(conn, WT_CONN_CACHE_POOL) &&
    (ret = __wt_config_gets(session, cfg, "cache_size", &cval)) == 0)
        conn->cache_size = (uint64_t)cval.val;
                        
if (F_ISSET(conn, WT_CONN_CACHE_POOL) &&
    (ret = __wt_config_gets(session, cfg,
    "shared_cache.reserve", &cval)) == 0 && cval.val != 0)
        cache->cp_reserved = (uint64_t)cval.val;
else if ((ret = __wt_config_gets(session, cfg,
    "shared_cache.chunk", &cval)) == 0)
        cache->cp_reserved = (uint64_t)cval.val;
WT_RET_NOTFOUND_OK(ret);
```
and pretty much all of `__wt_cache_pool_config`.  I'll try and catch up with you tonight and talk those over.

Most of the time we pass around a WT_SESSION_IMPL, so I think that should be the default.

And, where I really don't like passing a WT_CONNECTION_IMPL is when the called function immediately does `session = conn->default_session`.  That's making an assumption about the session we're using and I'd much rather see `S2C` than `session = conn->default_session`.

@michaelcahill, @agorrod copied for alternate opinions.