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)