Implement new RS1/RS2 handling according to RFC6189bis

RFC6189bis requires new handling of RS1 and RS2, see section 4.6.1 of that
RFC.
Fix NULL pointer access

The stop functions may be called twice, thus check for null pointers.
Modify RS2 valid/invalid handling

RS2 is set to invlaid by default. Only if some problems occur during the first
few packt of the SRTP media data the client may set it to valid to enable ZRTP
cache synchronization/self-healing.
Merge branch 'newchache' into develop
Do not report ZRTP CRC errors on every occurance

According to RFC 6189 CRC erros should not raise a warning. Thus remove the
waring if we see only a few CRC erros. If we see more CRC errors during a ZRTP
negotiation we raise a warning.
Merge branch 'hotfixCrc' into develop
Implement a first ZRTP tunnel via SDES/SRTP

If both clients support SDES we setup and addiitonal SRTP crypto contexts with
different KDF parameters. We use these SRTP contexts to protect the ZRTP
packets.

This commit still has debug messages and uses a specific ZRTP magic handling
to be backward compliant.
Implement signaling interface functions to support ZRTP encapsulation

Remove the test and debug code and use the signaling functions to enable or
disable ZRTP encapsulation. The SIP/SDP stack uses these funtions to get or
set the relevant attributes.
Merge branch 'tunnel' into develop