Fix deletion of VCA with slaved controls.

Fix deletion of VCA with slaved controls.

The crashed previously because:
 A VCA is-a Automatable is-a Evoral::ControlSet

 After VCA's d'tor completes ~Automatable runs and emits signal to
 DropReferences of all master-controls.

 This in turn calls SlavableAutomationControl::master_going_away()
 for slaved parameters for the given master-control

 In ::master_going_away() the weak-pointer reference to the master's
 AutomationControl (owned by the destroyed VCA) is still valid.

 Execution is in the d'tor of Automatable which is-a ControlSet and
 the ContolSet keeps a reference to the Control and hence also to the
 AutomationControl which is-a Evoral::Control.

 So master_going_away() locks a boost::shared_ptr<ARDOUR::AutomationControl>
 which is actually the MuteControl owned by the VCA.
 It calls SlavableAutomationControl::remove_master() which
 in turn calls MuteControl::pre_remove_master() which uses the
 MuteMaster API to retrieve the value. The MuteMaster however is the
 VCA that has just been destroyed.

The solution is twofold:
 1) emit "drop_references" from the VCA d'tor itself,
    before the VCA is destroyed.

 2) disconnect a slaved control from the master's drop_references signal
    when un-assigning a master-control.