vim-Import-JS can update wrong buffer if import-js is slow

When ImportJS is slow (e.g. I didn't have watchman installed when I created this issue and I had a large mono-repo with a lot of dependencies), vim-import-js can behave badly in 2 ways:

1. It can update the buffer AFTER other changes have been made to it (somewhat acceptable, I can imagine it's harder to know if the buffer has updated)
2. It can update another buffer if a user has moved to this offer buffer by the time the results from import-js come back (which is really weird)