ReadMsgHeader use bytes pool

[IIvyPy]

I find the makeslice of ReadMsgHeader will cost much GC time. So I suggest to use bytes pool to avoid the GC cost.

[miekg]

your commit message also don't explain anything about this change, it's just a one-liner

So it's on the client you want to use a buffer just like we do on the server side? Any data available (that should be in the ocmmit msg) on why and how this came about??

[tmthrgd]

I'm sympathetic to the performance concerns here, but this isn't quite the way to go about it. We'd need a pool on the Client struct itself. But considering there's been no movement here in almost a year, I'm going to go ahead and close this PR. Feel free to re-open and address the issues above.