bson: reduce DefaultBufferSize from 16K to 1K

The default buffer size is actually too high for generally average
payloads. Reducing to a more reasonable 1K.

benchmark                    old ns/op     new ns/op     delta
BenchmarkMarshal             26888         16526         -38.54%
BenchmarkUnmarshal           18394         18568         +0.95%
BenchmarkEncodeField         4852          4963          +2.29%
BenchmarkEncodeInterface     2964          3136          +5.80%

For bsonrpc, we already override this value to be 4K. So, this shouldn't
affect our performance.