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