Fix written rows metric in column writer

[sadikovi]

This PR addresses a problem in column writer when writing nullable non-repeated values. In this case the number of rows is the number of definition levels (at most number of values), but we were passing the actual number of values to write.

I also removed if statements in the loops in the hopes of improving performance.

Added test that verifies the correctness of the fix.

[sadikovi]

@sunchao Can you review this PR? Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 593

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 11 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 95.507%

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Files with Coverage Reduction	New Missed Lines	%
encodings/encoding.rs	1	94.82%
data_type.rs	1	93.09%
column/writer.rs	9	94.71%
<!--	Total:	11		-->

Totals
Change from base Build 591:	-0.01%
Covered Lines:	11967
Relevant Lines:	12530

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💛 - Coveralls