Introduce `MDB_IDL_LOGN` flags to heed

[Kerollmops]

It would be great to modify this LMDB derive to change the way LMDB behaves. You can read more on the implications here.

• [x] Reintroduce the feature flags.
• [x] Make the flag take the largest value instead of aborting
• [x] Either fork or wait for a patch to ifndef the MDB_IDL_LOGN define. Another solution could also be to sed the file in place when running the build.rs or apply this patch.

  cat ifndef-mdb-idl-logn.patch | patch -p0 lmdb/libraries/liblmdb/midl.h