raft: disambiguate promise name in raft::active_read
raft: disambiguate promise name in raft::active_read

gcc complains tha the name 'promise' changes meaning (from type
to variable) within active_read. Help it by disambiguating the
use as type.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla