databases/lmdb: add migration notice for 0.9.x -> 1.0.0 upgrade

LMDB 1.0 introduced an incompatible on-disk file format; v0.9 databases
must be exported with mdb_dump and re-imported with mdb_load before use.
Add a pkg-message (install always, upgrade only from <1.0.0) and an
UPDATING entry documenting the required migration procedure.

Suggested by: Kevin Bowling <kevin.bowling@kev009.com>