the same way as BTR_INSERT and BTR_DELETE_MARK: only perform (buffer)
the operation when the page is not in the buffer pool.

BTR_INSERT, BTR_DELETE_MARK, BTR_DELETE: Make the documentation say that
these flags are ignored when the page is in the buffer pool.

enum row_search_result: Remove ROW_NOT_DELETED, which
BTR_CUR_DELETE_FAILED was mapped to.

enum btr_cur_method: Remove BTR_CUR_DELETE_FAILED.  The
btr_cur_search_to_nth_level() will no longer attempt to execute the
BTR_DELETE when the page is in the buffer pool.

row_search_index_entry(): Remove the mapping from
BTR_CUR_DELETE_FAILED to ROW_NOT_DELETED.  The caller will have to
attempt purge when the record is in the buffer pool.

row_purge_remove_sec_if_poss_leaf(): Attempt to purge the record if it
was found in the buffer pool.

This addresses Issue #466.
rb://268