Scan multiple rows for determining type in strict mode

[evetion]

Alternative to #348.

Instead of trying to find a good default type for when the first row is NULL in strict mode, this PR scans the whole column until it finds a non-NULL value and uses that to derive the type. This not only fixes the issue described in #348, but also the downside of #346 that would corrupt Julia types in strict mode for columns starting with NULL (i.e. a symbol becoming a string).

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.25%. Comparing base (95131f8) to head (f9624b9).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/SQLite.jl	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
- Coverage   94.13%   93.25%   -0.89%     
==========================================
  Files           4        4              
  Lines         563      623      +60     
==========================================
+ Hits          530      581      +51     
- Misses         33       42       +9     

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[visr]

Thanks, yes this seems better to me than #348.