traversing the index B-tree twice (first in BTR_WATCH_LEAF mode and
then in BTR_DELETE mode), let BTR_DELETE take care of checking that
the record can be purged, and either buffering or performing the
purge.

row_purge_poss_sec(): New function, to check if it is possible to
purge a secondary index record.  Refactored from
row_purge_remove_sec_if_poss_low().

row_purge_remove_sec_if_poss_nonbuffered(): Rename to
row_purge_remove_sec_if_poss_tree().  Remove the parameter mode
(always use BTR_MODIFY_TREE).  Use row_purge_poss_sec().

row_purge_remove_sec_if_poss_low(): Rename to
row_purge_remove_sec_if_poss_leaf().  Remove the parameter mode
(always use BTR_MODIFY_LEAF). Let row_search_index_entry() do all the
hard work.

btr_cur_t: Add purge_node, which will be needed by
btr_cur_search_to_nth_level() for BTR_DELETE.  Replace the flag value
BTR_CUR_ABORTED with BTR_CUR_DELETE_REF and BTR_CUR_DELETE_FAILED.

enum row_search_result, row_search_index_entry(): Replace
ROW_NOT_IN_POOL with ROW_NOT_DELETED_REF and ROW_NOT_DELETED.

btr_cur_search_to_nth_level(): Remove BTR_WATCH_LEAF.  As a side
effect, the adaptive hash index can be used in purge as well.  If
BTR_DELETE cannot be buffered, attempt btr_cur_optimistic_delete().
Either way, check row_purge_poss_sec().  Move the code to set
cursor->ibuf_count to get rid of another if (height == 0)
check. Eliminate the label loop_end.  Do not call ibuf_should_try()
twice.

ibuf_should_try(): Now that the successful calls to this function will
be halved, halve the magic constant that ibuf_flush_count will be
compared to, accordingly.

The changes regarding ibuf_should_try() were merged from branches/zip
r3515.

rb://60 approved by Heikki over IM