Add share-server-delete API

This API is useful when share-server autodeletion is disabled, which is default behaviour.

Changes:
- added new api method with deletion of share server
- added db method, that takes list of shares by share server id
- changed logic of share manager, that is related to concurrency issues
- added unit and tempest tests to cover changes

Partially-implements blueprint add-admin-api-delete-share-server

Change-Id: I03b452de2cd4fe34c648b2434dab1b9244b1b005