We changed the hard-coded merge threshold (BTR_CUR_PAGE_COMPRESS_LIMIT
(50%))
to be settable for each index.

* merge threshold : If the data amount in the page becomes below the
threshold
when deleting row or updating to shorter row, attempts to merge with
neighbor page.

Even if the page was merged,
the page become near 100% filled and might cause split page soon.
If the threshold is set lower, the merged page farther from 100% and
might not cause
split page soon. Reducing opportunity for page merge/split benefit
performance.

New syntax: "MERGE_THRESHOLD=" specifying in comment clause of DDL is
parsed
(capital sensitive; should be capital letter)

Reviewed-by: Vasil Dimov <vasil.dimov@oracle.com>
Reviewed-by: Annamalai Gurusami <annamalai.gurusami@oracle.com>

RB: 7434 (5154 for bzr)