Eliminate InstanceIdentifierContext

Eliminate InstanceIdentifierContext

Rework the way we lookup strategies and generally resolve ApiPath to
actual request paths.

The primary logic now rests with RestconfStrategy, which has a way to
recursively deal with yang-ext:mount steps, resulting in a strategy +
ApiPath tail.

These are then picked up by MdsalRestconfServer and routed to the target
strategy's actual method -- which then interprets the tail ApiPath as
appropriate.

JIRA: NETCONF-1157
Change-Id: I8a6391f5538ba5662bad1bef79186e9a1e08ae81
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>