Cursor random

The next_random configuration to the WT_SESSION::open_cursor method configures the cursor to return a pseudo-random record from a row-store object.

The ability to return a random record was added to support a particular application, and as a result has somewhat unusual semantics. First, the returned record may not be random at all in the case of objects with only a few rows (especially when the object has never been written to the backing store). In such objects, the WT_CURSOR::next method for cursors configured with next_random may return the same row on each call. Additionally, even in larger objects, the WT_CURSOR::next method usually returns the first record from a random page in the underlying file, not a random record from a random page.