Using b:changedtick for change detection

For now, doing it just for detecting whether a change was made on move in insert
mode.

Using b:changedtick instead of our homebrew way of detecting the changed should
be both faster and more robust.
Only parsing file if changes detected

Previously we would do it on every CursorHold, buffer changed or not.

Fixes #485.
Revert to old way of tracking insert mode change

b:changedtick is incremented by the feedkeys() call which screws us up and
there doesn't seem to be a way around it that's worth the hassle.
Deleting some dead code
Ensure g:ycm_changedtick is always set.

Fixes #493.
Merge branch 'dev'