Reuse a bytes.Reader in shadowsocksWriter
Reuse a bytes.Reader in shadowsocksWriter

This avoids a memory allocation in every call to Write.

Master:
    BenchmarkTCPThroughput-4   	   64716	     18246 ns/op	       438 mbps	      81 B/op	       2 allocs/op

This branch:
    BenchmarkTCPThroughput-4   	   68146	     17924 ns/op	       446 mbps	      33 B/op	       1 allocs/op

Jigsaw-Code/outline-ss-server