swarm/bmt: async section writer interface to BMT (#778)

- AsyncHasher implements AsyncWriter interface
 - add extra level for zerohashes in pool to lookup empty data hash
 - remove unused segment, hash and depth fields from Tree
 - Hash pkg function -> syncHash moved to test
 - add asyncHash helper func to tests using shuffle
 - add TestAsyncCorrectness to tests
 - add BenchmarkBMTAsync to tests
 - refactor benchmarks using subbenchmarks
 - improved comments
 - preinitialise base hashers on the nodes