benchmark Sign and Verify operations

[btc]

#525 proposes signatures for DHT Provides records. Wanted to make sure that the runtime cost is well understood and that feasibility can be evaluated based on empirical evidence.

BenchmarkSign1B             1000           2406924 ns/op
BenchmarkSign10B            1000           2360527 ns/op
BenchmarkSign100B           1000           2354136 ns/op
BenchmarkSign1000B           500           2361775 ns/op
BenchmarkSign10000B          500           2415993 ns/op
BenchmarkSign100000B         500           2718051 ns/op
BenchmarkVerify1B          30000             52117 ns/op
BenchmarkVerify10B         30000             50968 ns/op
BenchmarkVerify100B        30000             51037 ns/op
BenchmarkVerify1000B       30000             56768 ns/op
BenchmarkVerify10000B      10000            105744 ns/op
BenchmarkVerify100000B      2000            628530 ns/op

[jbenet]

Yep, good to have these numbers present. 👍