* swarm/network, swarm/storage: validate default chunk size

* swarm/bmt, swarm/network, swarm/storage: update BMT hash initialisation

* swarm/bmt: move segmentCount to tests

* swarm/chunk: change chunk.DefaultSize to be untyped const

* swarm/storage: add size validator

* swarm/storage: add chunk size validation to localstore

* swarm/storage: move validation from localstore to validator

* swarm/storage: global chunk rules in MRU