The current implementation is to try to flush the neighbors of every
page that we flush. This patch makes the following distinction:

1) If the flush is from flush_list AND
2) If the flush is intended to move the oldest_modification LSN ahead
(this happens when a user thread sees little space in the log file and
attempts to flush pages from the buffer pool so that a checkpoint can
be made)

THEN

Do not try to flush the neighbors. Just focus on flushing dirty pages at
the end of flush_list

Approved by: Heikki