records on a page, count each unique buffered record at most once.

ibuf_get_volume_buffered_hash(): New function, for inserting a hashed record
into a bit array.

ibuf_get_volume_buffered_count(): New function, refactored from
ibuf_get_volume_buffered(), for counting the unique records buffered for
the page.

ibuf_get_volume_buffered(): Define hash_bitmap, a bit array for identifying
duplicate records.  Hash collisions do not hurt correctness, only performance.
If the number of records on the page is estimated as 0, deletes will not be
buffered.

This patch is just for the record, as is rb://44.  In the tests run so far,
the function ibuf_get_volume_buffered() always seems to estimate the number
of records as 0, disabling all buffered deletes.