This pr changes UnsafeForEach to traverse on boltdb before on the buffer.
This ordering guarantees that UnsafeForEach traverses in the same order
before or after the commit of buffer.
ReadTxs are designed for read-only accesses to the backend using a
read-only boltDB transaction. Since BatchTx's are long-running
transactions, all writes to BatchTx will writeback to ReadTx, overlaying
the base read-only transaction.
