(ITS#8704) LMDB add option to use prev meta page

[hyc@openldap.org]

Full_Name: Howard Chu
Version: LMDB 0.9
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (83.136.45.220)
Submitted by: hyc


Generally, running LMDB in NOSYNC mode means all integrity guarantees are gone,
in even of a system crash. However, if some other thread runs mdb_env_sync
periodically it's possible that the older DB state is still consistent, even if
slightly out of date. I have always intended to add an env flag for using the
older meta page, to potentially access a DB even after the latest txn snapshot
was corrupted.

A patch for this has been contributed, although support in the command-line
tools still needs to be added.