(ITS#7725) LMDB readahead should be configurable

[hyc@OpenLDAP.org]

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.