cmd, discovery, server: allow B to set MaxBroadcastPrice to filter acceptable O's stored in cache

[kyriediculous]

What does this pull request do? Explain your changes. (required)
Allows a broadcaster to set a maximum price preference BroadcastPrice *MaxBroadcastPrice (not required) in the same format as #916 either by specifying the flags maxPricePerUnit and pixelsPerUnit on node startup, or through the CLI.

When orchestratorPool.getOrchestrators is called on the orchestrators cached in the DBcache, it will filter orchestrators based on this max price setting (if set).

Specific updates (required)

• Added maxPricePerUnit flag to cmd/livepeer.go
• Added setting maxPricePerUnit and pixelsPerUnit through broadcaster CLI config
• Added a predicate function to orchestratorPool struct
• Added a new constructor to create an orchestratorPool with a predicate function NewOrchestratorPoolWithPred
• Adjusted DBOrchestratorPoolCache.getOrchestrators to initiate orchestratorPool with a predicate function through the new constructor NewOrchestratorPoolWithPred. The predicate function returns true when B's MaxBroadcastPrice isn't set, or is greater than Orchestrators PriceInfo
• Adjusted exported variable in server package BroadcastPrice to be a struct of type MaxBroadcastPrice consisting out of a *big.Rat and sync.RWMutex

How did you test each of these updates (required)
Added unit tests (80.3 % coverage for discovery package)
Added test to ./test_args.sh for when flags are set , but incorrectly provided

Does this pull request close any open issues?
Fixes #906

Checklist:

• README and other documentation updated
• Node runs in OSX and devenv
• All tests in ./test.sh pass

[yondonfu]

LGTM after a rebase!