bondingManager: make DLL in place representation of active set

[kyriediculous]

What does this pull request do? Explain your changes. (required)

• Make the sorted doubly linked list a fluent representation of the active transcoder set as stake updates happen
• Repurpose transcoder Registered status to indicate self-bonding as a willingness to be a transcoder
• Keep track of total stake for the active transcoder set

Specific updates (required)

• Removed setNumTranscoders() function
• Added helpers to increase/decrease stake for active transcoders as well as process total active set stake updates
• Added a helper to join the active set based on current transcoder stake
• Reworked resign method to do the necessary state updates for active transcoders
• Reworked isRegisteredTranscoder to indicate self-bonding
• Removed transcoderStatus() method, instead we now have isActiveTranscoder() and isRegisteredTranscoder() that return boolean values
• Deprecated numActiveTranscoders variable, instead we can get the size/maxSize of transcoderPool with the public helpers
• added a call to bondingManager.setCurrentRoundTotalActiveStake() upon round init to set nextRoundTotalActiveStake to the currentRoundTotalActiveStake
• adjusted unit & integration tests
• added a npm run script npm run test:describe to run tests for describe blocks (uses mocha's grep feature with -g flag)
• added a map lastActiveStakeUpdateRound that keeps track of the round in which an active transcoder's stake was last updated

How did you test each of these updates (required) Adjusted & ran unit and integation tests

Does this pull request close any open issues? Fixes #300 Fixes #302 Fixes #304

Checklist:

• [ ] README and other documentation updated
• [x] All unit & integration tests pass

[coveralls]

Pull Request Test Coverage Report for Build 812

• 74 of 75 (98.67%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.09%) to 99.528%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
contracts/bonding/BondingManager.sol	69	70	98.57%
<!--	Total:	74	75	98.67%	-->

Totals
Change from base Build 783:	-0.09%
Covered Lines:	749
Relevant Lines:	751

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💛 - Coveralls

[kyriediculous]

Added a commit that includes the requested changes as well as a new flag on the Transcoder struct, activationRound.

We can't do an active transcoder check on Transcoder.lastActiveStakeUpdateRound >0 && <= currentRound, because eg. when a delegate bonds to an already registered transcoder then the transcoder's lastActiveStakeUpdateRound will be set to currentRound + 1, resulting in isActiveTranscoder returning false when it should be true for the remainder of the round.

[yondonfu]

Since #323 is merged and these changes were included in that PR can we close this?

[kyriediculous]

Changed included in #323