This commit changes how ember-pouch implements `Adapter.createRecord`,
which is invoked after calling `.save()` on a new record,
so that it does not create multiple records if `.save()` is called
more than once before the first operation has finished.

If `.save()` is only invoked once before the record has finished persisting
to the DB (i.e. the promise that it returns has resolved) then
the behavior is unchanged. However, subsequent calls will wait for
the previously returned promise to resolve and then, if changes have
been made to the record, as indicated by Snapshot#changedAttributes,
it will delegate the task to `updateRecord`.

To avoid a problem caused by ember-data changing the ID associated
with the internalModel/record when the record has finished persisting,
the `Adapter.generateIdForRecord` method has been implemented so
that the ID is available immediately. Previously ember-pouch had still
been generating this id during `createRecord`, but ember-data was
not being made aware of this until its returned promise resolved.

Also, rather than rely on `adapter.db.rel.uuid()` to generate
an RFC4122 v4 UUID (requiring initialization to have completed),
this has been replaced by the equivalent `uuid` module from npm,
and the ember-auto-import addon has been installed to make it
easy to access this from within ember-pouch.

Finally, the `engines` section of package.json has been updated
to align with ember-auto-import's minimum version of 6.x
BREAKING CHANGE: drop node 4.x support (and 6.x/7.x not tested by CI)