Explicit allow to enable / disable session cache

Motivation:

It is sometimes useful to enable / disable the session cache.

Modifications:

* Add OpenSslSessionContext.setSessionCacheEnabled(...) and isSessionCacheEnabled()

Result:

It is now possible to enable / disable cache on the fly
Allow to set the context for which sessions can be used.

Motivation:

Openssl supports the SSL_CTX_set_session_id_context function to limit for which context a session can be used. We should support this.

Modifications:

Add OpenSslServerSessionContext that exposes a setSessionIdContext(...) method now.

Result:

It's now possible to use SSL_CTX_set_session_id_context.
Allow to override how headers are encoded

Motivation:

Even if its against the HTTP RFC there are situations where it may be useful to use other chars then US_ASCII in the headers. We should allow to make it possible by allow the user to override the how headers are encoded.

Modifications:

- Add encodeHeaders(...) method and so allow to override it.

Result:

It's now possible to encode headers with other charset then US_ASCII by just extend the encoder and override the encodeHeaders(...) method.
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