gbproxy: convert nse->bvcs from llist_head to hashtable

For the common lookup-by-bvci, this should reduce the computational
complexity significantly.

Depends: libosmocore.git I8ef73a62fe9846ce45058eb21cf999dd3eed5741
Change-Id: Ic8e9279fd61a3c514fc3203429f36a468f0e81d3
7 files changed