internal/transport: optimize grpc-message encoding/decoding

[tklauser]

Use strings.Builder (introduced in Go 1.10) instead of bytes.Buffer. Also call fmt.Fprintf directly on the builder. This improves performance both in terms of CPU and memory consumption:

name                 old time/op    new time/op    delta
DecodeGrpcMessage-8     345ns ± 5%     225ns ± 4%  -34.56%  (p=0.000 n=20+20)
EncodeGrpcMessage-8    1.33µs ± 4%    1.26µs ± 3%   -5.76%  (p=0.000 n=20+20)

name                 old alloc/op   new alloc/op   delta
DecodeGrpcMessage-8     80.0B ± 0%     24.0B ± 0%  -70.00%  (p=0.000 n=20+20)
EncodeGrpcMessage-8      115B ± 0%       88B ± 0%  -23.48%  (p=0.000 n=20+20)

name                 old allocs/op  new allocs/op  delta
DecodeGrpcMessage-8      2.00 ± 0%      2.00 ± 0%     ~     (all equal)
EncodeGrpcMessage-8      8.00 ± 0%      4.00 ± 0%  -50.00%  (p=0.000 n=20+20)

RELEASE NOTES: none

[dfawley]

This is definitely a micro-optimization but it's no more complicated than the existing code and seems to be more of a best practice than the bytes.Buffer so LGTM. Thanks for the PR!

[zasweq]

LGTM.