(ITS#7725) LMDB readahead should be configurable

Full_Name: Howard Chu Version: 2.4.35+ OS: Linux URL: ftp://ftp.openldap.org/incoming/ Submission from: (NULL) (78.155.233.73) Submitted by: hyc

In 2.4.35 (commit b1da555c4c7275d7f756d693c42814388a9aa839) code was added to unconditionally turn off readahead, because it is harmful to random read performance when a DB is larger than RAM and/or system RAM is already full. Unfortunately turning it off harms sequential read performance. It should be configurable, since different behaviors are needed in different use cases.