Adding int-to-object map implementation.

Motivation:

Maps with integer keys are used in several places (HTTP/2 code, for
example). To reduce the memory footprint of these structures, we need a
specialized map class that uses ints as keys.

There are a few libraries out there that do this, but they all seem to
be fairly heavy weight, so it wouldn't be desirable to take a dependency
on them for a couple of classes.

Modifications:

I've based this implementation on the Trove project (which is LGPL),
which in turn bases the PrimeFinder logic by work done at CERN. We
probably need to discuss the implications of this from a legal
perspective.

The IntObjectHashMap is the main class and it uses PrimeFinder to find
the next prime > 2*currentCapacity. If needed, we could simply replace
the PrimeFinder logic with something like "capacity *= 2".

The rest of the IntObjectHashMap is based on the work of Trove's
TIntObjectHashMap (a generated class). Much of the code has been
modified, but the core algorithm for open addressing remains very close
to the original.

Result:

A new int-based map class that can be used to increase
memory/performance throughout Netty.
I think this is a bad idea as if the user still has a reference to the previous Entry you will cause side-effects on it when update the entry.index.  So a new Entry should be created.