db_bench: new benchmark - seek to deleted ranges

[Yuval-Ariel]

Owner: yuval

We need to test the performance when the user does delete of large range of keys (one after the other) and than calls seek to the first element in this range.

The test is:

create N ranges where each range is M long. - hash the prefix of each range so as not to be sequential in db. the range itself is sequential. 

then start deleting the ranges while simultaneously adding more ranges (same ranges as before). do this in the same thread, delete a whole range, then write a new range.

after deleting K ranges, start seeking to the beginning of the deleted ranges and time it. while still creating and deleting ranges.