messaging: choose connection index via a look-up table
messaging: choose connection index via a look-up table

Looking up is faster than a bunch of if()s.

scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla