inspiral.py: a few tweaks to coinc counting

- aesthetics: in appsink_new_buffer(), when histogramming likelihood ratios for zero-lag candidates, don't inline the construction of the instruments tag for the coinc
- in __flush():  when counting non-coincident singles, do not count singles recovered during single-instrument time.  this test is already part of appsink_new_buffer() but was forgotten here
- performance:  in __flush():  when histogramming likelihood ratios for zero-lag candidates, rely on the zero_lag_time_slide_ids set maintained by stream_thinca.last_coincs to identify all-zero offset vectors;  construct the instruments tag inside the conditional to skip when not needed