bonding: Support address + amount params in withdrawFees

[yondonfu]

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

This PR adds support for specifying a recipient address and amount in withdrawFees() in the BondingManager. This feature is required for https://github.com/livepeer/protocol/issues/495.

This is a breaking change to the withdrawFees() function and WithdrawFees() event API and will require the following updates:

• go-livepeer calls to withdrawFees()
• explorer calls to withdrawFees()
• subgraph indexing of the WithdrawFees() event

The rationale for this breaking change:

• The BondingManager is already really large
• The update to support this breaking change is small

May want to consider updating slimming down the BondingManager and TicketBroker further by removing *for* functions in favor of accepting args, but we can consider that after completing the current set of tasks to get to feature complete milestone.

Specific updates (required)

See commit history.

How did you test each of these updates (required)

Updated tests.

Does this pull request close any open issues?

Fixes #503

Checklist:

• [x] README and other documentation updated
• [x] All tests using yarn test pass

[kautukkundan]

How about we add a test case validating this order of events?

1. claims fees partially
2. more fees is rewarded
3. claims again (fully or partially)

and check if the saved/claimed fees is correct.

[yondonfu]

How about we add a test case validating this order of events?

Added the following test cases in dd72a12:

• Full withdrawal (to complement the existing partial withdrawal test case)
• Withdrawal after earning more fees

Will rebase to resolve conflicts after the above commit is reviewed.