Refactor code.

Make it easier to read and prevent future uses of the bandwidth argument to
NewIOThrottlerPool.
Simplify tests by using assertNil and assertNotNil.
go fmt
More simplifying of the unit tests.

Moved the addr argument within the function. It never varied.
Simplification of the unit tests.

Handle an error within createTcpPipe so it's callers don't have to handle it.