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>
Stop using global variables as prefixes to inject the writer header.
That can cause issues when two writers set the length of the buffer in
the same header concurrently.
Stop Writing to the internal buffer twice for each write. This could
mess up with the ordering information is written.
Signed-off-by: David Calavera <david.calavera@gmail.com>
