(ITS#9027) LMDB: API to retrieve map address

[github@nicwatson.org]

Full_Name: Nic Watson
Version: 
OS: Linux
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (73.132.68.128)


Goal:
I'd like a clean way to get at the address of the data memory map in LMDB.

MDB_envinfo.me_mapaddr only returns the map address if MAP_FIXED is used.

Current Workarounds:
* Use OS-specific mechanism to retrieve all memory maps (e.g.
/proc/<pid>/smaps). 
* Defeat opaque handle and reach into the MDB_env struct directly and grab the
me_map field.

Justification:
In my current application, I notice a significant performance increase if I
mlock the mapfile.  In order to do that cleanly, I need the address of the map.