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]

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 ⚠️

Additional details and impacted files

@@            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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