vinyl: introduce vy_run_writer

vy_run_writer incapsulates logic of writing statements to a run.
It provides API to write statements one by one. It is needed so
that we can abort a run writing task before waiting for it to
finish writing the file.

Edited by @locker:
 - add region_truncate() wherever necessary
 - do not reallocate row index ibuf for each page
 - remove vy_run_write(), use vy_run_writer directly instead

Needed for #3166