Fixed major bug EXT-6869 (Viewer crashes when switching between group voice sessions)

The reason: LLCallDialogManager did not disconnect from "voice channel state changed" signal on channel in previous session.
    Thus when old channel changes state from STATE_HUNG_UP to STATE_CALL_STARTED there is invalid pointer to session in connected LLCallDialogManager::onVoiceChannelStateChanged callback before current voice channel is changed and pointer to session becames valid.

Fix: add storing of connection to signal in voice channel from previous session and disconnect it before connecting to channel in current session.

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/244/

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branch : product-engine