* proxy: add benchmark
Signed-off-by: Tw <tw19881113@gmail.com>

* replacer: prepare lazily

update issue#939

benchmark            old ns/op     new ns/op     delta
BenchmarkProxy-4     83865         72544         -13.50%
Signed-off-by: Tw <tw19881113@gmail.com>

* proxy: use buffer pool to avoid temporary allocation
Signed-off-by: Tw <tw19881113@gmail.com>