effective_replication_map holds the full replication_map
resulting from applying the effective replication strategy
over the given token_metadata and replication_strategy_config_options.
It is calculated once, in make_effective_replication_map(), and then it
can be used for retrieving the endpoints/token_ranges synchronously
from the precalculated map.
A new virtual get_natural_endpoints(const token&, const effective_replication_map&)
method has been added to abstract_replication_strategy so that
local_strategy and everywhere_replication_strategy can override it as they may be
needed before the token_metadata is established.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>