Common cache serializer and json serializer

[maca88]

This PR adds a new shared library NHibernate.Cache.Common, proposed by @fredericDelaporte which allows to share the new ICacheSerializer interface among distributed cache providers and also adds a NHibernate.Caches.Util.JsonSerializer library containing a json serializer by using Json.NET.
I've tested the json serializer with the NHiberante tests successfully (master branch), by configuring the serializer as:

_serializer = new JsonCacheSerializer();
_serializer.RegisterType(typeof(string[]), "sa");
_serializer.RegisterType(typeof(NHSpecificTest.NH2392.PhoneNumber), "pn");
_serializer.RegisterType(typeof(NHSpecificTest.NH2394.PhoneNumber), "pn2");
_serializer.RegisterType(typeof(NHSpecificTest.NH3874.IntWrapper), "iw");
_serializer.RegisterType(typeof(NHSpecificTest.EntityWithUserTypeCanHaveLinqGenerators.BarExample), "bar");
_serializer.RegisterType(typeof(NHSpecificTest.EntityWithUserTypeCanHaveLinqGenerators.FooExample), "foo");
_serializer.RegisterType(typeof(NullableInt32), "nint", jsonContract =>
{
  // Use fields instead of properties
  var contract = (JsonObjectContract) jsonContract;
  contract.Properties.Clear();
  var properties = typeof(NullableInt32).GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance)
                      .Select(f => new JsonProperty
                      {
                        PropertyName = f.Name,
                        PropertyType = f.FieldType,
                        DeclaringType = f.DeclaringType,
                        ValueProvider = new ReflectionValueProvider(f),
                        AttributeProvider = new ReflectionAttributeProvider(f),
                        Readable = true,
                        Writable = true

                      });
  foreach (var property in properties)
  {
    contract.Properties.AddProperty(property);
  }
}, typeof(JsonObjectContract)); // Because of the TypeConverter attribute, the default resolved json contract for NullableInt32 is JsonStringContract.

TODO:

• Wait for the NHibernate 5.2 release and update the code

[fredericDelaporte]

I have rebased this. The only updates required for compiling were unrelated obsolete fixes.

Had you in mind other changes to do?

[maca88]

Had you in mind other changes to do?

Yes there are some TODOs in this PR that I intend to do. I'm going to do them now.

[maca88]

Done all the changes that I planned to do.