Provide safe access to LMDB version

[martinvonwittich]

I'm currently playing around with accessing a shared LMDB database from multiple languages (Perl, Python, PHP, Rust, NodeJS), and I'm getting an MDB_VERSION_MISMATCH error in my NodeJS implementation. To further debug this, I tried to determine the LMDB version that each implementation is using, which worked...

• in NodeJS:

   let lmdb = require('node-lmdb');
   console.log(lmdb.version);
  

• in Python:

   import lmdb
   print("{}".format(lmdb.version()))
  

• in Perl:

   use LMDB_File qw(:flags :version);
   printf "%s\n", LMDB_File::MDB_VERSION_STRING;
  

• in PHP:

   printf("%s\n", dba_handlers(true)["lmdb"]);
  

But I can't find a way to access this information from Rust with the lmdb crate. As far as I can tell, the only way to get it is by using mdb_version from lmdb-sys, but that seems to be unsafe, and as a Rust newbie, I don't dare use it. From the lmdb-sys code it looks like as if the current version is 0.9.21, but that should probably be queryable via code :)