Fix regression on out of sync cursors.

- Fix 1 regression where cursors could potentially get out of sync in insert
  mode
- Added some logic to debug latency. Fanning out to 30 cursors in insert mode
  with my vimrc took over 300ms. It's like than 20ms with a plain vimrc. Need to
  debug what setting is causing the slowing down in insert mode and inform
  users.
Merge branch 'master' of github.com:terryma/vim-multiple-cursors