Disable automatic tags file updates during :vimgrep

This change is related to pull request #83 however that pull request
wasn't merged here (and won't be merged at all) because it was based on
the old/dead `async-cleanup' feature branch (see pull request #49 on
GitHub) instead of the new `async-take-two' feature branch (see pull
request #84 on GitHub). This change set implements the equivalent on the
new feature branch (without introducing another option).
Disable automatic tags file updates during session loading
Don't use :redraw in :HighlightTags during session loading