Big tables performance optimizations

This commit does not change table rendering engine of vimwiki,
instead it optimizes duplicate calls / unnecessary line changes
etc.:

1. do not reset &tw if s:textwidth was 0
2. do not setline() rows that did not change
3. call s:get_rows() and vimwiki#tbl#get_cells() only once

These changes make inserting new rows in Insert mode in tables of
approx. 100 lines length about 4 times faster in my tests