Refine SQLFS reader

[sneaxiy]

• Remove ut TestSQLFSHiveWriterWriteAndRead and TestSQLFSSQLWriterWriteAndRead, since they are the same as TestSQLFSWriteAndRead.
• Make sqlfs.reader return 0, io.EOF at the end of the table, which behaves the same as os.Open(...) in Go.

[codecov-io]

Codecov Report

Merging #3100 (3e9197c) into develop (c9d48b1) will decrease coverage by 48.45%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop   #3100       +/-   ##
===========================================
- Coverage    50.88%   2.43%   -48.46%     
===========================================
  Files          248     165       -83     
  Lines        18447   11397     -7050     
===========================================
- Hits          9387     277     -9110     
- Misses        8364   11075     +2711     
+ Partials       696      45      -651     

Impacted Files	Coverage Δ
python/runtime/temp_file.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/pai/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/local/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/model/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/db_writer/mysql.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/feature/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/xgboost/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/verifier/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/verifier/verifier.py	0.00% <0.00%> (-100.00%)	:arrow_down:
python/runtime/db_writer/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 203 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update c9d48b1...3e9197c. Read the comment docs.