* [vnet] app proxying
* Make uri.AppendLeafCluster noop if name is empty
This makes it easier to converting profileName + leafName combos into
URIs.
* Make sure daemon.Service.NewClusterClient sets MFAPromptConstructor
* auth -> authclient
* remove unnecessary changes in teleport storage
* plumb clock and test cert refreshes
* s/zeebo/testify
* fix tcp echo handler
* only login to one cluster at a time
* add godoc comments in clientcache
* test cert expiry, serialize clock advances
* add comments on relogin mutex
* typo fix
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Co-authored-by: Rafał Cieślak <rafal.cieslak@goteleport.com>
rosstimothy