Switch from nremoved_ tracking to modstate_.

nremoved_ was a correct way to ensure that Masstree's version number
changed at the right times to avoid bugs, when accessing Masstree
via its get/put/scan interface. But Silo needs to access Masstree
in an additional way, via full_version_value(). We want the following
invariant: If the set of keys in a leaf changes, then its
full_version_value() also changes. But a delete followed by an insert
might not change full_version_value()! So ensure that any transition
between "inserting" state and "removing" state changes full_version_
value(). Found with Nathaniel Herman.