Acquire BlockRing lock during submitting request to ring and caching on
internal InFlight list.
Also remove some of the Frontend's indirection calls by exposing the
BlockRing, Notifier and Granter, and allowing direct calls to member
functions

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Track current/maximum grants used

Signed-off-by: Owen Smith <owen.smith@citrix.com>