Added possibility to forward errors

[tosan88]

Changes include:

• An app could specify an error channel as a parameter for new consumer which would now receive the errors while consuming from kafka (also kept existing behavior as well)
• Updated vendoring for kazoo-go lib
• A new consumer now is created via config struct (better for extending constructor params)

NOTE: No tests for the new feature & some data race is detected by CircleCI in the underlying library

• added tests for the new feature, refactored existing tests a bit
• cannot reproduce the data race, tests run cleanly, both locally and on CircleCI
• removed unused err return value from ftmessage#parseHeaders and adjusted callers

[coveralls]

Coverage increased (+2.5%) to 95.273% when pulling 2a65348 on improvements into af40f10 on master.

[tosan88]

👍 for the changes by @tamas-molnar

[kovacshuni]

yeah!