Rename __cursor_search_clear __cursor_pos_clear; it's the code that
clears a cursor's location, and is no longer unique to the search
functions.

Change __cursor_error_resolve cursor error resolution to clear the
cursor's location even if we see a failure from the cursor "leave"
function (probably a bad return from releasing a page), keeping the
cursor's location isn't likely to make anything better.

sename __cursor_error_resolve to __cursor_reset, it's a combination of
"leave the API" and "clear the cursor's location"; now that it clears
the cursor location regardless of the return from the cursor "leave"
function, we can call it from the WT_CURSOR.reset code, they're the
same.
update
Clean up __cursor_pos_clear: only clear the fields we need to clear and
initialize the other fields consistently in the search functions (for
example, slot is always set by row-store search, make it always set by
column-store search as well).

This isn't a performance change (although it can't hurt).  There are
already many fields we don't clear in __cursor_pos_clear because it's
too expensive to clear them -- this just makes it consistent, we clear
what we have to clear and ignore the rest.
Merge branch 'row-search-slim' into row-search-slim-fast-append-first