Reuse encoding buffer

[rakitzis]

Reuse request encoding buffer across sends
conn.encodeBuf is retained to encode outgoing request packets.
conn.m is held for the entire write sequence so serialized
access is guaranteed.

This shaves an allocation.

Before:

BenchmarkRoundTrip/Plain/1KB-10         	  329035	      3706 ns/op	 276.34 MB/s	    1627 B/op	       6 allocs/op
BenchmarkRoundTrip/Plain/64KB-10        	  128247	      9145 ns/op	7166.00 MB/s	   74212 B/op	       6 allocs/op
BenchmarkRoundTrip/Plain/1MB-10         	   22280	     53276 ns/op	19682.07 MB/s	 1057258 B/op	       6 allocs/op
BenchmarkRoundTrip/Encrypted/1KB-10     	  241368	      5028 ns/op	 203.67 MB/s	    3554 B/op	       7 allocs/op
BenchmarkRoundTrip/Encrypted/64KB-10    	   39223	     30939 ns/op	2118.22 MB/s	  164387 B/op	       7 allocs/op
BenchmarkRoundTrip/Encrypted/1MB-10     	    3859	    306180 ns/op	3424.71 MB/s	 2377340 B/op	       7 allocs/op

After:

BenchmarkRoundTrip/Plain/1KB-10         	  286184	      3561 ns/op	 287.58 MB/s	    1500 B/op	       5 allocs/op
BenchmarkRoundTrip/Plain/64KB-10        	  129012	      9022 ns/op	7264.18 MB/s	   74084 B/op	       5 allocs/op
BenchmarkRoundTrip/Plain/1MB-10         	   22854	     52175 ns/op	20097.33 MB/s	 1057221 B/op	       5 allocs/op
BenchmarkRoundTrip/Encrypted/1KB-10     	  240336	      4948 ns/op	 206.95 MB/s	    3426 B/op	       6 allocs/op
BenchmarkRoundTrip/Encrypted/64KB-10    	   39441	     30362 ns/op	2158.48 MB/s	  164319 B/op	       6 allocs/op
BenchmarkRoundTrip/Encrypted/1MB-10     	    3963	    307307 ns/op	3412.14 MB/s	 2376662 B/op	       6 allocs/op