update dependencies to pull in large-cluster gossip performance improvements

[rboyer]

The bulk of the changes are in hashicorp/memberlist#167 with a minor satellite change in hashicorp/serf#545.

The github.com/hashicorp/go-sockaddr update is related only in that it is a transitive dependency of memberlist that was updated during testing to rectify some go modules plumbing issues.

[pearkes]

@rboyer I think this still needs the vendorfmt treatment here:

consul/GNUmakefile

Lines 123 to 126 in 067346e

	vendorfmt:
	@echo "--> Formatting vendor/vendor.json"
	test -x $(GOPATH)/bin/vendorfmt || go get -u github.com/magiconair/vendorfmt/cmd/vendorfmt
	vendorfmt

That will limit the diff on the vendor file.

[rboyer]

TIL make vendorfmt

[pearkes]

Target revisions and vendor format look good 👍