Fix streaming in versions of node > 12

• Fixes issue described in #120 where stats stream stops reporting

Fix: Deal with exception when err is undefined.

• Will now gracefully handle an undefined error.

Rename slave circuits

slaveCircuits have been renamed to subCircuits to help erase prejudicial language.

Disable error prefixing

• Added a toggle to disabled prefixing error message with circuit name

Cumulative Statistics

• Added cumulative counters that are independent of buckets. These counter stream out total requests (timeouts, etc.) per breaker, as well as the diff between two report events.

Added circuit generations

• Adds in the concept of circuit generations. A generation will be an integer that increments by one every time a circuit is opened. Service requests will be tagged with a generation and the generation will be checked on failed response to make sure we don't inadvertently pollute the current circuit with requests that are from a previous circuit generation.
• Add the breaker name to circuit rejections

Fix callback detection to support single argument anonymous functions.

• Updates regex detection for callback detection when passing a function

Fix Bug with Fallback when Using Slave Circuits

• Fixes issue found in #78. Slave circuits weren't using master fallback when circuit was open.

Include error in timeout and failure event

• The timeout and failure event now provides the underlying error as the second argument.

isPromise and Error Percentage Fix

• Added isPromise and isFunction options to Brakes. These options will force the library to treat functions that are passed in as either promises or callbacks.
• Fixed an issue where errorPercentage in the hystrixStream was not being aggregated by turbine correctly due to being a float