Add `nrepl-switch-to-relevant-repl-buffer' as separate functionality.

Commit 12558e8 breaks `nrepl-switch-to-repl-buffer' functionality by
assuming that all nrepl connections will be made using
`nrepl-jack-in'.

This commit reverts to old `nrepl-switch-to-repl-buffer'. A new
function `nrepl-switch-to-relevant-repl-buffer' is added as a
convenience function to switch to the correct nrepl buffer from a
given clojure buffer.