leveldb: Use native Darwin memory barriers
leveldb: Use native Darwin memory barriers

Earlz/dobbscoin-source,dobbscoin/dobbscoin-source,Earlz/dobbscoin-source,dobbscoin/dobbscoin-source,Earlz/dobbscoin-source,Earlz/dobbscoin-source,dobbscoin/dobbscoin-source,Earlz/dobbscoin-source,dobbscoin/dobbscoin-source,dobbscoin/dobbscoin-source,Earlz/dobbscoin-source,dobbscoin/dobbscoin-source