Use a buffered writer to improve performance

[theunrepentantgeek]

Improve the performance of the export files stage by buffering file writes using buffio.

Rudimentary/naïve benchmark:

Unbuffered - 193.7s
Buffered - 123.1s

Saved time ~70s (~35%)

(Benchmark method: one run under each condition without isolating the effect of other processes on the machine.)

[coveralls]

Coverage decreased (-1.7%) to 65.478% when pulling e0132b6 on improve/write-buffers into 7c7cd93 on master.

[babbageclunk]

Very nice!