* raft: use file paths for TLS info in the retry_join stanza
* raft: maintain backward compat for existing tls params
* docs: update raft docs with new file-based TLS params
* Update godoc comment, fix docs
* adding support for TLS 1.3 for TCP listeners
* removed test as CI uses go 1.12
* removed Cassandra support, added deprecation notice
* re-added TestTCPListener_tls13
* use observer pattern for service discovery
* update perf standby method
* fix test
* revert usersTags to being called serviceTags
* use previous consul code
* vault isnt a performance standby before starting
* log err
* changes from feedback
* add Run method to interface
* changes from feedback
* fix core test
* update example
Go 1.13 flipped TLS 1.3 to opt-out instead of opt-in, and its TLS 1.3
support does not allow configuring cipher suites. Simply remove the
affected test; it's not relevant going forward and there's ample
evidence it works properly prior to Go 1.13.
