BUG-8327: GlobalBundleScanningSchemaServiceImpl should be a proxy

We are currently running to separate services which assemble
the GlobalSchemaContext, which hurts our startup performance and
leads to wasted memory. This is an artefact of the mdsal split,
hence we should be getting the service from the MD-SAL and
just proxy to old interfaces.

This lowers the startup time for

feature:install odl-restconf odl-bgpcep-bgp
    odl-bgpcep-data-change-counter odl-netconf-topology

from 86s down to 67s (22%). Final retained heap size is also
lowered from 217MiB to 181MiB (16%)

Change-Id: I549e9512538bd83d86cfd2164d03e34bc9130c1e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>