Switching macOS builds of dfx to LMDB backend (experimental)

[dist1ll]

dfx will be using the LMDB artifact pool backend in a future release. If you encounter any issues like crashes or instability, please let us know. You'll still be able to use the previous backend by starting dfx with the --rocksdb flag.

Background: dfx can tell the replica binary which database backend to use for storing artifacts. On macOS, by default we have been using RocksDB. However, the RocksDB backend has not been used by the IC in production for some time now, so we want to phase it out completely.