Remove keystore loading code from handshakers

@mkovatsc can you please review my refactoring because it starts to be quite aggressive :)

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The client and server handshaker class was loading keystore and
truststore. I removed this code, the store need to be given at
DTLSConnector creation.

It's removing clutter from the handshaker classes and help to understand
how/who/where the key are loaded (single responsibility principle)

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I removed ScProperties because  it's a quite opiniated way to handle configuration.
A lot of Java programmers already have their configuration Fwk (OSGi config admin, 
JEE or Spring stuff..). So I think it's more idiomatic to provide setter/getter on the main API 
classes. The constructor started to grow so I made differents methods for configuring the
 server and client parts of the connector.