agent: Rename src→clientConn and dst→targetConn to be clearer
agent: Rename src→clientConn and dst→targetConn to be clearer

This should clear up a confusing error message that a user got.

Signed-off-by: Luke Shumaker <1abe3165bcc8c29e8b31e5c88636540fb9350004@datawire.io>

datawire/telepresence,datawire/telepresence,datawire/telepresence