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

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other thread runs mdb_env_sync periodically it's possible that the older DB state is still consistent, even if slightly out of date.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.