Fix overriden method not called in TrafficShaping

Motivation:
handlerAdded and handlerRemoved were overriden but super was never
called, while it should.
Also add one missing information in the toString method.

Modifications:
Add the super corresponding call, and add checkInterval to the
toString() method

Result;
super method calls are correctly passed to the super implementation
part.
Improve error reporting in SingleThreadEventLoopTest. Fixes #2894

Motivation:

When an assertTrue(condition) statement fails we usually don't know
why, as the parameters of the condition are not logged.

Modifications:

Include relevant parameters in the assertion error message.

Result:

Easier to debug and understand test failures.
Change HTTP/2 tests to retain rather than copy ByteBufs

Motivation:

The current build is showing potential leaks in the HTTP/2 tests that
use Http2TestUtil.FrameCountDown, which copies the buffers when it
receives them from the decoder. The leak detecor sees this copy as the
source of a leak. It would be better all around to just retain, rather
than copying the buffer. This should help to lower the overall memory
footprint of the tests as well as potentially getting rid of the
reported "leaks".

Modifications:

Modified Http2TestUtil to use ByteBuf.retain() everywhere that was
previously calling ByteBuf.copy().

Result:

Smaller memory footprint for tests and hopefully getting rid of reported
leaks.
Updating HTTP/2 tests to not retain buffers for validation

Motivation:

To eliminate the tests as being a cause of leaks, removing the automatic
retaining of ByteBufs in Http2TestUtil.

Modifications:

Each test that relied on retaining buffers for validation has been
modified to copy the buffer into a list of Strings that are manually
validated after the message is received.

Result:

The HTTP/2 tests should (hopefully) no longer be reporting erroneous
leaks due to the testing code, itself.