Create Default Serializer via a Factory method

Thus it is configurable from outside which Serializer and with
which configuration is actually used as default.
Merge branch 'compatible_factotory' of github.com:marcust/memcached-session-manager into marcust-compatible_factotory
Rename sys prop to msm.kryo.defaultSerializerFactory
Rename kryo sys prop names *.buffersize.* to *.bufferSize.*

To be a more consistent.

In detail, these are changed:

* msm.kryo.buffersize.initial -> msm.kryo.bufferSize.initial
* msm.kryo.buffersize.max -> msm.kryo.bufferSize.max