couch_mrview: couch_mrview_changes:handle_changes

Similar to couch_changes:handle_changes but for view changes. It add
support for longpolling, normal and continuous stream The API differs
from the one for doc by beeing independant from the transport: the
support of HTTP will be added on top for example.

This API will be also used to replace the view filter in the current _changes
API.

Also add unittests.