flytable_base4table too conservative listing tables

[jefferis]

• It can spend 45s listing tables 4x out of a 60s seatable cell types query ...
• this may still be overkill (rescanning if a table can't be found) but probably safest compromise

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6d88e89) 54.38% compared to head (dbea404) 54.37%.

Files	Patch %	Lines
R/flytable.R	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #197 +/- ## ========================================== - Coverage 54.38% 54.37% -0.02% ========================================== Files 38 38 Lines 4773 4776 +3 ========================================== + Hits 2596 2597 +1 - Misses 2177 2179 +2 ```

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