Replace StreamID with an opaque bigint

Nobody has claimed this yet.

For a Livepeer node, this corresponds to the JobID, from which the StreamID can be looked up easily on-chain. Other off-chain applications can use the bigint as a lookup key.

Currently, the only time StreamID is used concretely within basicnet is to determine the broadcaster's NodeID. We should be able to work around this. Treat the ID as an opaque identifier that's used only by the API consumer, not basicnet.

Benefits:

* The StreamID right now is not a canonical identifier; we need to maintain a reverse lookup table for the JobID to find any transcoder information. This is additional bookkeping that we should avoid.
* Concretely, solves this issue https://github.com/livepeer/go-livepeer-basicnet/issues/21#issuecomment-369310870
* Doesn't make assumptions about the structure of the StreamID within basicnet. This reduces coupling with Livepeer -- other systems might not want to make the same assumptions about their bookkeeping.
* Further decouples basicnet from Livepeer; keeping this identifier opaque means any changes to the ID in Livepeer doesn't necessitate changes in basicnet.

No file or test path is named. Start by tracing StreamID references in basicnet, especially the logic that determines the broadcaster's NodeID, and inspect issue #21 for context; done means the identifier is an opaque bigint and basicnet no longer depends on its internal structure or reverse lookups.

Written by the indexing model from the issue text.