Add a lot of benchmarks

[slimsag]

This change adds a bunch of benchmarks to the various appdash data types. Currently on my machine this reports:

BenchmarkChunkedCollector500            1000       2447726 ns/op
BenchmarkMarshalEvent                 100000         16036 ns/op
BenchmarkUnmarshalEvents               50000         35877 ns/op
BenchmarkIDGeneration               10000000           166 ns/op
BenchmarkFlatten                      200000          7431 ns/op
BenchmarkNewRootSpanID               5000000           340 ns/op
BenchmarkNewSpanID                  10000000           176 ns/op
BenchmarkSpanIDString                 200000          6694 ns/op
BenchmarkParseSpanID                 2000000           702 ns/op
BenchmarkMemoryStore100                 3000        555790 ns/op
BenchmarkMemoryStore250                 2000       1517543 ns/op
BenchmarkMemoryStore1000                 300       5345421 ns/op
BenchmarkMemoryStoreWrite1000            200       6092708 ns/op
BenchmarkMemoryStoreReadFrom1000         100      11200728 ns/op
BenchmarkRecentStore500                  500       4793635 ns/op