As a first step towards using host_id to identify nodes instead of ip addresses
this series introduces a node abstraction, kept in topology,
indexed by both host_id and endpoint.
The revised interface also allows callers to handle cases where nodes
are not found in the topology more gracefully by introducing `find_node()` functions
that look up nodes by host_id or inet_address and also get a `must_exist` parameter
that, if false (the default parameter value) would return nullptr if the node is not found.
If true, `find_node` throws an internal error, since this indicates a violation of an internal
assumption that the node must exist in the topology.
Callers that may handle missing nodes, should use the more permissive flavor
and handle the !find_node() case gracefully.
Closes#11987
* github.com:scylladb/scylladb:
topology: add node state
topology: remove dead code
locator: add class node
topology: rename update_endpoint to add_or_update_endpoint
topology: define get_{rack,datacenter} inline
shared_token_metadata: mutate_token_metadata: replicate to all shards
locator: endpoint_dc_rack: refactor default_location
locator: endpoint_dc_rack: define default operator==
test: storage_proxy_test: provide valid endpoint_dc_rack
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