Remove CID lists

[dirkmc]

Supersedes #216
Fixes #178
Fixes immediate bottleneck in #166 (but there is another bottleneck)

TODO:

• Unit tests for new cid list methods
• Figure out what to do about channel state migration at https://github.com/filecoin-project/go-data-transfer/blob/master/channels/internal/migrations/migrations.go#L67
  Resolution: deprecate CID lists and ignore migration - it's 7 months old so everyone should have migrated by now
• Benchmarking (see investigate low performance and CPU hotspot transferring 4GiB over loopback #166)

[aarshkshah1992]

@dirkmc Some comments.