Support for disabling conversation mode

[rosecitytransit]

Mentioned in #791 and is a follow up to #326

I've thought the recording mode should be at least a 3 way option.

Upon termination flag (transmission end):

• call is always closed, putting each transmission in a separate call (future transmissions are started based upon their grant messages by ignoring check for existing call)
• (current mode) file is closed but call is kept open
• file is kept open and closed on call end

To do conversation mode only involves:

1. not closing the file in end_transmission() here

   trunk-recorder/trunk-recorder/gr_blocks/transmission_sink.cc

   Line 224 in e33e441

   close_wav(false);

2. adding close_wav() to stop_recording()

   trunk-recorder/trunk-recorder/gr_blocks/transmission_sink.cc

   Line 257 in e33e441

   void transmission_sink::stop_recording() {

   since end_transmission() will no longer do it
3. having a separate sample counter so that close_wav() creates a correct header
4. Ignoring or removing the error messages about the file not closing/being open
5. modifying call concluder to handle the entire call being in the first transmission file
6. Also need to move

   trunk-recorder/trunk-recorder/gr_blocks/transmission_sink.cc

   Lines 546 to 547 in ca64f7a

   	d_stop_time = time(NULL);
   	d_last_write_time = std::chrono::steady_clock::now();

   outside of transmission_sink::dowork because on a termination flag they will not be reached due to

   trunk-recorder/trunk-recorder/gr_blocks/transmission_sink.cc

   Line 483 in ca64f7a

   return noutput_items;

Originally posted by @rosecitytransit in #791 (comment)

[robotastic]

Keeping the current approach - it is a good balance of keeping the number of files reasonable and not having to move file pointers around too. Happy to reconsider if there is a strong desire for this

[robotastic]

Actually... maybe we should enable some support for turning off conversation mode.... I will leave this open