Creates a `fixedBuffer` type that is used to encapsulate functionality
for reading/writing from the underlying byte slices.
Uses lazily-loaded set of sync.Pools for storing buffers that are no
longer needed so they can be re-used.
```
benchmark old ns/op new ns/op delta
BenchmarkBytesPipeWrite-8 138469 48985 -64.62%
BenchmarkBytesPipeRead-8 130922 56601 -56.77%
benchmark old allocs new allocs delta
BenchmarkBytesPipeWrite-8 18 8 -55.56%
BenchmarkBytesPipeRead-8 0 0 +0.00%
benchmark old bytes new bytes delta
BenchmarkBytesPipeWrite-8 66903 1649 -97.54%
BenchmarkBytesPipeRead-8 0 1 +Inf%
```
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Turn BytesPipe's Read and Write functions into blocking, goroutine-safe
functions. Add a CloseWithError function to propagate an error code to
the Read function.
Adjust tests to work with the blocking Read and Write functions.
Remove BufReader, since now its users can use BytesPipe directly.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
