New option to reuse buffers when appending records

[Yves57]

If you have similar record lists to append to a file, reusing some internal slices saves memory and seems to significantly improve the performances.
On my computer:

Configuration
ReuseBuffers=false Deflate=false	81_858_921 ns/op	137_463_702 B/op	10_341 allocs/op
ReuseBuffers=true_Deflate=false	19_568_794 ns/op	21_023_761 B/op	10_051 allocs/op
ReuseBuffers=false_Deflate=true	261_185_675 ns/op	145_642_182 B/op	10_520 allocs/op
ReuseBuffers=true_Deflate=true	122_100_900 ns/op	9_794_948 B/op	10_223 allocs/op

The only strange thing is that reusing buffers takes less memory with deflate than without compression (but it is slower, which is consistent).

Do you think that it would interesting to add a such option?

[yuzic]

may be in one style ?
var bb = bytes.NewBuffer(deflateBlock)

[xmcqueen]

Hi are you still there? Are you still interested in discussing this PR?

[Yves57]

@xmcqueen Yes I'm still there. Do you have any remark/question?