cache ipns entries to speed things up a little

core/commands/ipns.go

core/commands/publish.go

core/core.go

fuse/ipns/ipns_test.go

namesys/namesys.go

namesys/pb/namesys.pb.go

namesys/pb/namesys.proto

namesys/publisher.go

namesys/republisher/repub_test.go

namesys/resolve_test.go

namesys/routing.go

repo/config/init.go

repo/config/ipns.go

test/sharness/t0240-republisher.sh