nsqd: benchmark various write/msg sizes

[Dieterbe]

because this has a big impact on performance

caveats:

• if /tmp runs full, you'll get unrealistically high results,
  can be avoided by running with -benchtime=0.1s
• for some reason on my system QueueGet > 1024 hangs

on my system:
BenchmarkDiskWrite16      300000               465 ns/op          34.36 MB/s
BenchmarkDiskWrite64      300000               476 ns/op         134.30 MB/s
BenchmarkDiskWrite256     300000               521 ns/op         491.03 MB/s
BenchmarkDiskWrite1024    200000               634 ns/op        1614.39 MB/s
BenchmarkDiskWrite4096    200000              1060 ns/op        3860.56 MB/s
BenchmarkDiskWrite16384    50000              2672 ns/op        6131.59 MB/s
BenchmarkDiskWrite65536    20000             10629 ns/op        6165.22 MB/s
BenchmarkDiskWrite262144            3000             39678 ns/op        6606.64 MB/s
BenchmarkDiskWrite1048576           1000            162506 ns/op        6452.50 MB/s
BenchmarkDiskWriteBuffered16    10000000                27.4 ns/op       583.05 MB/s
BenchmarkDiskWriteBuffered64     5000000                30.3 ns/op      2113.78 MB/s
BenchmarkDiskWriteBuffered256    1000000               108 ns/op        2367.44 MB/s
BenchmarkDiskWriteBuffered1024    500000               346 ns/op        2958.68 MB/s
BenchmarkDiskWriteBuffered4096    200000              1170 ns/op        3500.34 MB/s
BenchmarkDiskWriteBuffered16384    50000              2746 ns/op        5965.97 MB/s
BenchmarkDiskWriteBuffered65536    10000             10374 ns/op        6316.90 MB/s
BenchmarkDiskWriteBuffered262144            3000             41288 ns/op        6349.07 MB/s
BenchmarkDiskWriteBuffered1048576           1000            162323 ns/op        6459.78 MB/s

BenchmarkDiskQueueGet16   200000               956 ns/op          16.73 MB/s
BenchmarkDiskQueueGet64   200000              1021 ns/op          62.68 MB/s
BenchmarkDiskQueueGet256          100000              1252 ns/op         204.47 MB/s
BenchmarkDiskQueueGet1024         100000              2104 ns/op         486.49 MB/s

[mreiferson]

Thanks! Looks like this needs a rebase

[Dieterbe]

rebased.