If a B elects their O, reduce the B's reserve requirement

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**Is your feature request related to a problem? Please describe.**

In order to procure transcoding services on Livepeer Public Network, a Broadcaster must maintain sufficient reserve deposit to cover the face value of a winning probabilistic micropayment ticket for each of the 100 Orchestrators in the active set.

For example, if the face value of a ticket is 0.1 ETH, then a Broadcaster must put up a reserve of 10 ETH (around $30,000 at current prices).

This represents a barrier-to-entry for new participants into the Broadcaster space.

**Describe the solution you'd like**

I would like for the Livepeer Broadcaster software to be able to a) elect to work with only one (1) Orchestrator, **_and_** b) to only be required to maintain sufficient reserve deposit to cover the face value of a winning probabilistic micropayment ticket for one (1) Orchestrator.

At the time of writing, it is possible to configure a Livepeer Broadcaster to a) elect to work with only one (1) Orchestrator (using the `-orchAddr` flag or `-ethOrchAddr` flag), but the B is still required to maintain a reserve to cover 100 winning tickets.

**Describe alternatives you've considered**

To continue to wait until another project attempts to compete with livepeer.com in running Broadcaster infrastructure.

**Additional context**

Related to https://github.com/livepeer/go-livepeer/issues/2262

Start by tracing how the broadcaster handles reserve deposits and how the -orchAddr and -ethOrchAddr options affect orchestrator selection. Determine how the active orchestrator count should affect the required reserve, then validate the behavior with the relevant broadcaster and payment tests.

Written by the indexing model from the issue text.