String column decoder reserves much larger memory than needed

[yibin87]

Bug Report

When decode columns, we use IColumn's reserve(size_t rows) interface to avoid reallocating memories each time. However, for string columns, the element size is not fixed, and we used 64 bytes as default value. This will lead to much larger memory usage when actual element size is much smaller than 64 bytes.
This problem will be worsen for Join operator, since all build side's blocks will be in memory for a while.