tests: compare crud and vshard performance

This patch adds new cases for performance tests: select for equal
conditions for primary and secondary indexes (including known bucket_id
case from #220), and adds corresponding vshard test cases to compare
performance. Comparison may be not exactly precise since vshard test
functions use naive mergers, but should at least estimate basic
differences.

Test run on HP ProBook 440 G7 i7/16Gb/256SSD shows that CRUD module is
4-8 times slower than vshard calls for prepared functions for select and
2 times slower for insert.

Closes #225