Avoids allocations and copying by using a buffer pool for intermediate
writes.
```
benchmark old ns/op new ns/op delta
BenchmarkWrite-8 996 175 -82.43%
benchmark old MB/s new MB/s speedup
BenchmarkWrite-8 4414.48 25069.46 5.68x
benchmark old allocs new allocs delta
BenchmarkWrite-8 2 0 -100.00%
benchmark old bytes new bytes delta
BenchmarkWrite-8 4616 0 -100.00%
```
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Upstream-commit: 2f3b6022858aab2d70e380d126e6dcec227c0d38
Component: engine
8a8affba81