views: actually write to the view, not the underlying store

This is (obviously) needed if we want to make big commits.

Signed-off-by: David Scott <dave.scott@citrix.com>
Coalesce all the 'destroy' and 'clear' side-effects together

We would like to present the updates to irmin in large batches
for efficiency.

Signed-off-by: David Scott <dave.scott@citrix.com>