Add integration tests that measure how throughput varies with latency.
Add integration tests that measure how throughput varies with latency.

This clearly shows the impact high latency networks have on the
throughput of sftp transfers. The current benchmark results on my
machine are:

$ go test -bench=. -integration
PASS
BenchmarkRead1k	      20	  93028268 ns/op	 112.72 MB/s
BenchmarkRead16k	     100	  14912438 ns/op	 703.16 MB/s
BenchmarkRead32k	     100	  12282661 ns/op	 853.71 MB/s
BenchmarkRead128k	     100	  10550935 ns/op	 993.83 MB/s
BenchmarkRead512k	     100	  12970518 ns/op	 808.44 MB/s
BenchmarkRead1MiB	     100	  11526693 ns/op	 909.70 MB/s
BenchmarkRead4MiB	     100	  15761135 ns/op	 665.30 MB/s
BenchmarkRead4MiBDelay10Msec	       1	2662626928 ns/op	   3.94 MB/s
BenchmarkRead4MiBDelay50Msec	       1	12977334971 ns/op	   0.81 MB/s
BenchmarkRead4MiBDelay150Msec	       1	38777294968 ns/op	   0.27 MB/s
BenchmarkWrite1k	       5	 235318220 ns/op	  44.56 MB/s
BenchmarkWrite16k	      50	  57347520 ns/op	 182.85 MB/s
BenchmarkWrite32k	      30	  66151560 ns/op	 158.51 MB/s
BenchmarkWrite128k	      20	  86642733 ns/op	 121.02 MB/s
BenchmarkWrite512k	      20	  72648883 ns/op	 144.34 MB/s
BenchmarkWrite1MiB	      20	  87145317 ns/op	 120.33 MB/s
BenchmarkWrite4MiB	      50	  64098344 ns/op	 163.59 MB/s
BenchmarkWrite4MiBDelay10Msec	       1	3360676836 ns/op	   3.12 MB/s
BenchmarkWrite4MiBDelay50Msec	       1	16321294488 ns/op	   0.64 MB/s
BenchmarkWrite4MiBDelay150Msec	       1	48731747397 ns/op	   0.22 MB/s
BenchmarkCopyDown10MiBDelay10Msec	       1	3369680836 ns/op	   3.11 MB/s
BenchmarkCopyDown10MiBDelay50Msec	       1	16228310257 ns/op	   0.65 MB/s
BenchmarkCopyDown10MiBDelay150Msec	       1	48505306499 ns/op	   0.22 MB/s
BenchmarkCopyUp10MiBDelay10Msec	       1	3393474907 ns/op	   3.09 MB/s
BenchmarkCopyUp10MiBDelay50Msec	       1	16273951972 ns/op	   0.64 MB/s
BenchmarkCopyUp10MiBDelay150Msec	       1	48461724780 ns/op	   0.22 MB/s
BenchmarkMarshalInit	 2000000	       779 ns/op
BenchmarkMarshalOpen	 2000000	       645 ns/op
BenchmarkMarshalWriteWorstCase	   20000	     70954 ns/op
BenchmarkMarshalWrite1k	  300000	      5072 ns/op
ok  	github.com/pkg/sftp	296.179s

jtwatson/sftp,DrJosh9000/sftp,akupas/sftp,fuchsia-mirror/third_party-github.com-pkg-sftp,pkg/sftp,fuchsia-mirror/third_party-pkg-sftp,ScriptRock/sftp