Revert "MB-10660 wrap update_vbucket_state into spawn_rebalance_subprocess"

This reverts commit 718ba6e2ef445454e2af89f4c38635e60f97d5df.

We found that it may break capi_set_view_manager state due to lack of
proper serialization.

Change-Id: I4e67ab07b8683ef23e7f537998de1d5b05ce999b
Reviewed-on: http://review.couchbase.org/35734
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>