perf(col_str): reduce max memory usage

[zdyj3170101136]

Summary

here is memory inuse bytes pprof.

the ColStr WriteColumn copy the buf.

reduce max memory usage bytes by use writev.

[zdyj3170101136]

@ernado cc

[zdyj3170101136]

@ernado could you merge it ?

[ernado]

Please address the question in review comments.

[vincentbernat]

I am unsure if a resolved conversation will be visible enough for people reviewing, hence this comment.

From my understanding, if c.Buf does not have enough space to store a varint, it is extended. The condition is hit on the very first use (and is already covered by unit tests). This is needed as now the representation inside c.Buf is, for each string, varint + string. c.Pos still represents the start and end of the string without the varint.